diff --git a/README.md b/README.md
index 1f37288..f382b34 100644
--- a/README.md
+++ b/README.md
@@ -333,6 +333,7 @@ MultiCurl::setProxyAuth($auth)
 MultiCurl::setProxyTunnel($tunnel = true)
 MultiCurl::setProxyType($type)
 MultiCurl::setRange($range)
+MultiCurl::setRateLimit($rate_limit)
 MultiCurl::setReferer($referer)
 MultiCurl::setReferrer($referrer)
 MultiCurl::setRetry($mixed)
diff --git a/src/Curl/MultiCurl.php b/src/Curl/MultiCurl.php
index 973bbad..83b4603 100644
--- a/src/Curl/MultiCurl.php
+++ b/src/Curl/MultiCurl.php
@@ -15,6 +15,16 @@ class MultiCurl
     private $concurrency = 25;
     private $nextCurlId = 0;
 
+    private $rateLimit = null;
+    private $rateLimitEnabled = false;
+    private $rateLimitReached = false;
+    private $maxRequests = null;
+    private $interval = null;
+    private $intervalSeconds = null;
+    private $unit = null;
+    private $currentStartTime = null;
+    private $currentRequestCount = 0;
+
     private $beforeSendCallback = null;
     private $successCallback = null;
     private $errorCallback = null;
@@ -713,6 +723,57 @@ class MultiCurl
         $this->setOpt(CURLOPT_RANGE, $range);
     }
 
+    /**
+     * Set Rate Limit
+     *
+     * @access public
+     * @param  $rate_limit string (e.g. "60/1m").
+     */
+    public function setRateLimit($rate_limit)
+    {
+        $rate_limit_pattern =
+            '/' .       // delimiter
+            '^' .       // assert start
+            '(\d+)' .   // digit(s)
+            '\/' .      // slash
+            '(\d+)?' .  // digit(s), optional
+            '(s|m|h)' . // unit, s for seconds, m for minutes, h for hours
+            '$' .       // assert end
+            '/' .       // delimiter
+            'i' .       // case-insensitive matches
+            '';
+        if (!preg_match($rate_limit_pattern, $rate_limit, $matches)) {
+            throw new \UnexpectedValueException(
+                'rate limit must be formatted as $max_requests/$interval(s|m|h) ' .
+                '(e.g. "60/1m" for a maximum of 60 requests per 1 minute)'
+            );
+        }
+
+        $max_requests = (int)$matches['1'];
+        if ($matches['2'] === '') {
+            $interval = 1;
+        } else {
+            $interval = (int)$matches['2'];
+        }
+        $unit = strtolower($matches['3']);
+
+        // Convert interval to seconds based on unit.
+        if ($unit === 's') {
+            $interval_seconds = $interval * 1;
+        } elseif ($unit === 'm') {
+            $interval_seconds = $interval * 60;
+        } elseif ($unit === 'h') {
+            $interval_seconds = $interval * 3600;
+        }
+
+        $this->rateLimit = $max_requests . '/' . $interval . $unit;
+        $this->rateLimitEnabled = true;
+        $this->maxRequests = $max_requests;
+        $this->interval = $interval;
+        $this->intervalSeconds = $interval_seconds;
+        $this->unit = $unit;
+    }
+
     /**
      * Set Referer
      *
@@ -823,17 +884,21 @@ class MultiCurl
         }
 
         $this->isStarted = true;
-
-        $concurrency = $this->concurrency;
-        if ($concurrency > count($this->curls)) {
-            $concurrency = count($this->curls);
-        }
-
-        for ($i = 0; $i < $concurrency; $i++) {
-            $this->initHandle(array_shift($this->curls));
-        }
+        $this->currentStartTime = microtime(true);
+        $this->currentRequestCount = 0;
 
         do {
+            while (count($this->curls) &&
+                count($this->activeCurls) < $this->concurrency &&
+                (!$this->rateLimitEnabled || $this->hasRequestQuota())
+            ) {
+                $this->initHandle();
+            }
+
+            if ($this->rateLimitEnabled && !count($this->activeCurls) && !$this->hasRequestQuota()) {
+                $this->waitUntilRequestQuotaAvailable();
+            }
+
             // Wait for activity on any curl_multi connection when curl_multi_select (libcurl) fails to correctly block.
             // https://bugs.php.net/bug.php?id=63411
             if (curl_multi_select($this->multiCurl) === -1) {
@@ -842,7 +907,7 @@ class MultiCurl
 
             curl_multi_exec($this->multiCurl, $active);
 
-            while (!($info_array = curl_multi_info_read($this->multiCurl)) === false) {
+            while (($info_array = curl_multi_info_read($this->multiCurl)) !== false) {
                 if ($info_array['msg'] === CURLMSG_DONE) {
                     foreach ($this->activeCurls as $key => $curl) {
                         if ($curl->curl === $info_array['handle']) {
@@ -868,10 +933,7 @@ class MultiCurl
                                 // Remove completed instance from active curls.
                                 unset($this->activeCurls[$key]);
 
-                                // Start new requests before removing the handle of the completed one.
-                                while (count($this->curls) >= 1 && count($this->activeCurls) < $this->concurrency) {
-                                    $this->initHandle(array_shift($this->curls));
-                                }
+                                // Remove handle of the completed instance.
                                 curl_multi_remove_handle($this->multiCurl, $curl->curl);
 
                                 // Clean up completed instance.
@@ -883,11 +945,7 @@ class MultiCurl
                     }
                 }
             }
-
-            if (!$active) {
-                $active = count($this->activeCurls);
-            }
-        } while ($active > 0);
+        } while ($active || count($this->activeCurls) || count($this->curls));
 
         $this->isStarted = false;
     }
@@ -993,8 +1051,17 @@ class MultiCurl
      * @param  $curl
      * @throws \ErrorException
      */
-    private function initHandle($curl)
+    private function initHandle()
     {
+        $curl = array_shift($this->curls);
+        if ($curl === null) {
+            return;
+        }
+
+        // Add instance to list of active curls.
+        $this->currentRequestCount += 1;
+        $this->activeCurls[$curl->id] = $curl;
+
         // Set callbacks if not already individually set.
         if ($curl->beforeSendCallback === null) {
             $curl->beforeSend($this->beforeSendCallback);
@@ -1033,7 +1100,57 @@ class MultiCurl
             throw new \ErrorException('cURL multi add handle error: ' . curl_multi_strerror($curlm_error_code));
         }
 
-        $this->activeCurls[$curl->id] = $curl;
         $curl->call($curl->beforeSendCallback);
     }
+
+    /**
+     * Has Request Quota
+     *
+     * Checks if there is any available quota to make additional requests while
+     * rate limiting is enabled.
+     *
+     * @access private
+     */
+    private function hasRequestQuota()
+    {
+        // Calculate if there's request quota since ratelimiting is enabled.
+        if ($this->rateLimitEnabled) {
+            // Determine if the limit of requests per interval has been reached.
+            if ($this->currentRequestCount >= $this->maxRequests) {
+                $elapsed_seconds = microtime(true) - $this->currentStartTime;
+                if ($elapsed_seconds <= $this->intervalSeconds) {
+                    $this->rateLimitReached = true;
+                    return false;
+                } elseif ($this->rateLimitReached) {
+                    $this->rateLimitReached = false;
+                    $this->currentStartTime = microtime(true);
+                    $this->currentRequestCount = 0;
+                }
+            }
+
+            return true;
+        } else {
+            return true;
+        }
+    }
+
+    /**
+     * Wait Until Request Quota Available
+     *
+     * Waits until there is available request quota available based on the rate limit.
+     *
+     * @access private
+     */
+    private function waitUntilRequestQuotaAvailable()
+    {
+        $sleep_until = $this->currentStartTime + $this->intervalSeconds;
+        $sleep_until_relative = $sleep_until - $this->currentStartTime;
+        $sleep_seconds = $sleep_until - microtime(true);
+
+        // Avoid using time_sleep_until() as it appears to be less precise and not sleep long enough.
+        usleep($sleep_seconds * 1000000);
+
+        $this->currentStartTime = microtime(true);
+        $this->currentRequestCount = 0;
+    }
 }
diff --git a/tests/PHPCurlClass/Helper.php b/tests/PHPCurlClass/Helper.php
index 02d2f52..bceb1a8 100644
--- a/tests/PHPCurlClass/Helper.php
+++ b/tests/PHPCurlClass/Helper.php
@@ -50,6 +50,16 @@ class Test
         $this->chainedRequest($first, $data);
         $this->chainedRequest($second, $data);
     }
+
+    public static function getTestUrl($port)
+    {
+        if (getenv('PHP_CURL_CLASS_LOCAL_TEST') === 'yes' ||
+            in_array(getenv('TRAVIS_PHP_VERSION'), array('7.0', '7.1', '7.2', '7.3', '7.4', 'nightly'))) {
+            return 'http://127.0.0.1:' . $port . '/';
+        } else {
+            return self::TEST_URL;
+        }
+    }
 }
 
 function create_png()
@@ -127,3 +137,11 @@ function remove_file_from_server($uploaded_file_path) {
     )));
     assert(file_exists($uploaded_file_path) === false);
 }
+
+function get_multi_curl_property_value($instance, $property_name)
+{
+    $reflector = new \ReflectionClass('\Curl\MultiCurl');
+    $property = $reflector->getProperty($property_name);
+    $property->setAccessible(true);
+    return $property->getValue($instance);
+}
diff --git a/tests/PHPCurlClass/PHPMultiCurlClassTest.php b/tests/PHPCurlClass/PHPMultiCurlClassTest.php
index 639c67d..4c2eaeb 100644
--- a/tests/PHPCurlClass/PHPMultiCurlClassTest.php
+++ b/tests/PHPCurlClass/PHPMultiCurlClassTest.php
@@ -3421,4 +3421,1138 @@ class MultiCurlTest extends \PHPUnit\Framework\TestCase
         });
         $multi_curl->start();
     }
+
+    public function testSetRateLimitUnits()
+    {
+        foreach (array(
+                array(
+                    'rate_limit' => '1/s',
+                    'expected' => array(
+                        'rate_limit' => '1/1s',
+                        'max_requests' => '1',
+                        'interval' => '1',
+                        'unit' => 's',
+                        'interval_seconds' => '1',
+                    ),
+                ),
+                array(
+                    'rate_limit' => '1/1s',
+                    'expected' => array(
+                        'rate_limit' => '1/1s',
+                        'max_requests' => '1',
+                        'interval' => '1',
+                        'unit' => 's',
+                        'interval_seconds' => '1',
+                    ),
+                ),
+                array(
+                    'rate_limit' => '10/60s',
+                    'expected' => array(
+                        'rate_limit' => '10/60s',
+                        'max_requests' => '10',
+                        'interval' => '60',
+                        'unit' => 's',
+                        'interval_seconds' => '60',
+                    ),
+                ),
+                array(
+                    'rate_limit' => '1/m',
+                    'expected' => array(
+                        'rate_limit' => '1/1m',
+                        'max_requests' => '1',
+                        'interval' => '1',
+                        'unit' => 'm',
+                        'interval_seconds' => '60',
+                    ),
+                ),
+                array(
+                    'rate_limit' => '1/1m',
+                    'expected' => array(
+                        'rate_limit' => '1/1m',
+                        'max_requests' => '1',
+                        'interval' => '1',
+                        'unit' => 'm',
+                        'interval_seconds' => '60',
+                    ),
+                ),
+                array(
+                    'rate_limit' => '5000/60m',
+                    'expected' => array(
+                        'rate_limit' => '5000/60m',
+                        'max_requests' => '5000',
+                        'interval' => '60',
+                        'unit' => 'm',
+                        'interval_seconds' => '3600',
+                    ),
+                ),
+                array(
+                    'rate_limit' => '1/h',
+                    'expected' => array(
+                        'rate_limit' => '1/1h',
+                        'max_requests' => '1',
+                        'interval' => '1',
+                        'unit' => 'h',
+                        'interval_seconds' => '3600',
+                    ),
+                ),
+                array(
+                    'rate_limit' => '1/1h',
+                    'expected' => array(
+                        'rate_limit' => '1/1h',
+                        'max_requests' => '1',
+                        'interval' => '1',
+                        'unit' => 'h',
+                        'interval_seconds' => '3600',
+                    ),
+                ),
+                array(
+                    'rate_limit' => '5000/1h',
+                    'expected' => array(
+                        'rate_limit' => '5000/1h',
+                        'max_requests' => '5000',
+                        'interval' => '1',
+                        'unit' => 'h',
+                        'interval_seconds' => '3600',
+                    ),
+                ),
+                array(
+                    'rate_limit' => '100000/24h',
+                    'expected' => array(
+                        'rate_limit' => '100000/24h',
+                        'max_requests' => '100000',
+                        'interval' => '24',
+                        'unit' => 'h',
+                        'interval_seconds' => '86400',
+                    ),
+                ),
+            ) as $test) {
+            $multi_curl = new MultiCurl();
+            $multi_curl->setRateLimit($test['rate_limit']);
+
+            $this->assertEquals(
+                $test['expected']['rate_limit'],
+                \Helper\get_multi_curl_property_value($multi_curl, 'rateLimit')
+            );
+            $this->assertEquals(
+                $test['expected']['max_requests'],
+                \Helper\get_multi_curl_property_value($multi_curl, 'maxRequests')
+            );
+            $this->assertEquals(
+                $test['expected']['interval'],
+                \Helper\get_multi_curl_property_value($multi_curl, 'interval')
+            );
+            $this->assertEquals(
+                $test['expected']['unit'],
+                \Helper\get_multi_curl_property_value($multi_curl, 'unit')
+            );
+            $this->assertEquals(
+                $test['expected']['interval_seconds'],
+                \Helper\get_multi_curl_property_value($multi_curl, 'intervalSeconds')
+            );
+        }
+    }
+
+    public function testSetRateLimitPerSecond1()
+    {
+        //  R0--|
+        //  R1--|
+        //      W---------------|
+        //                      R2--|
+        //                      R3--|
+        //                          W---------------|
+        //                                          R4--|
+        //  0   1   2   3   4   5   6   7   8   9   10  11  12
+
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('2/5s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=1');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=1');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=1');
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=1');
+        $multi_curl->addGet(Test::getTestUrl('8005') . '?seconds=1');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R0 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['0']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['0']['start_diff']);
+        // Assert R0 ends around 1.
+        $this->assertGreaterThanOrEqual(0.8, $request_stats['0']['stop_diff']);
+        $this->assertLessThanOrEqual(1.5, $request_stats['0']['stop_diff']);
+
+        // Assert R1 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['1']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['1']['start_diff']);
+        // Assert R1 ends around 1.
+        $this->assertGreaterThanOrEqual(0.8, $request_stats['1']['stop_diff']);
+        $this->assertLessThanOrEqual(1.5, $request_stats['1']['stop_diff']);
+
+        // Assert R2 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['2']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['2']['start_diff']);
+        // Assert R2 ends around 6.
+        $this->assertGreaterThanOrEqual(5.8, $request_stats['2']['stop_diff']);
+        $this->assertLessThanOrEqual(6.5, $request_stats['2']['stop_diff']);
+
+        // Assert R3 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['3']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['3']['start_diff']);
+        // Assert R3 ends around 6.
+        $this->assertGreaterThanOrEqual(5.8, $request_stats['3']['stop_diff']);
+        $this->assertLessThanOrEqual(6.5, $request_stats['3']['stop_diff']);
+
+        // Assert R4 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['4']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['4']['start_diff']);
+        // Assert R4 ends around 11.
+        $this->assertGreaterThanOrEqual(10.8, $request_stats['4']['stop_diff']);
+        $this->assertLessThanOrEqual(11.5, $request_stats['4']['stop_diff']);
+    }
+
+    public function testSetRateLimitPerSecond2()
+    {
+        //  R0--|
+        //  R1------|
+        //          W-----------|
+        //                      R2--|
+        //                      R3------|
+        //                              W-----------|
+        //                                          R4--|
+        //  0   1   2   3   4   5   6   7   8   9   10  11  12
+
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('2/5s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=1');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=2');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=1');
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=2');
+        $multi_curl->addGet(Test::getTestUrl('8005') . '?seconds=1');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R0 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['0']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['0']['start_diff']);
+        // Assert R0 ends around 1.
+        $this->assertGreaterThanOrEqual(0.8, $request_stats['0']['stop_diff']);
+        $this->assertLessThanOrEqual(1.5, $request_stats['0']['stop_diff']);
+
+        // Assert R1 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['1']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['1']['start_diff']);
+        // Assert R1 ends around 2.
+        $this->assertGreaterThanOrEqual(1.8, $request_stats['1']['stop_diff']);
+        $this->assertLessThanOrEqual(2.5, $request_stats['1']['stop_diff']);
+
+        // Assert R2 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['2']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['2']['start_diff']);
+        // Assert R2 ends around 6.
+        $this->assertGreaterThanOrEqual(5.8, $request_stats['2']['stop_diff']);
+        $this->assertLessThanOrEqual(6.5, $request_stats['2']['stop_diff']);
+
+        // Assert R3 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['3']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['3']['start_diff']);
+        // Assert R3 ends around 7.
+        $this->assertGreaterThanOrEqual(6.8, $request_stats['3']['stop_diff']);
+        $this->assertLessThanOrEqual(7.5, $request_stats['3']['stop_diff']);
+
+        // Assert R4 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['4']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['4']['start_diff']);
+        // Assert R4 ends around 11.
+        $this->assertGreaterThanOrEqual(10.8, $request_stats['4']['stop_diff']);
+        $this->assertLessThanOrEqual(11.5, $request_stats['4']['stop_diff']);
+    }
+
+    public function testSetRateLimitPerSecond3()
+    {
+        //  R0------|
+        //  R1------------------|
+        //                      R2------|
+        //                      R3------------------|
+        //                                          R4--|
+        //  0   1   2   3   4   5   6   7   8   9   10  11  12
+
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('2/5s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=2');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=5');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=2');
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=5');
+        $multi_curl->addGet(Test::getTestUrl('8005') . '?seconds=1');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R0 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['0']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['0']['start_diff']);
+        // Assert R0 ends around 2.
+        $this->assertGreaterThanOrEqual(1.8, $request_stats['0']['stop_diff']);
+        $this->assertLessThanOrEqual(2.5, $request_stats['0']['stop_diff']);
+
+        // Assert R1 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['1']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['1']['start_diff']);
+        // Assert R1 ends around 5.
+        $this->assertGreaterThanOrEqual(4.8, $request_stats['1']['stop_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['1']['stop_diff']);
+
+        // Assert R2 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['2']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['2']['start_diff']);
+        // Assert R2 ends around 7.
+        $this->assertGreaterThanOrEqual(6.8, $request_stats['2']['stop_diff']);
+        $this->assertLessThanOrEqual(7.5, $request_stats['2']['stop_diff']);
+
+        // Assert R3 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['3']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['3']['start_diff']);
+        // Assert R3 ends around 10.
+        $this->assertGreaterThanOrEqual(9.8, $request_stats['3']['stop_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['3']['stop_diff']);
+
+        // Assert R4 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['4']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['4']['start_diff']);
+        // Assert R4 ends around 11.
+        $this->assertGreaterThanOrEqual(10.8, $request_stats['4']['stop_diff']);
+        $this->assertLessThanOrEqual(11.5, $request_stats['4']['stop_diff']);
+    }
+
+    public function testSetRateLimitPerSecond4()
+    {
+        //  R0------|
+        //  R1----------------------------------|
+        //                      R2----------|
+        //                      R3----------------------|
+        //                                          R4--|
+        //  0   1   2   3   4   5   6   7   8   9   10  11  12
+
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('2/5s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=2');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=9');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=3');
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=6');
+        $multi_curl->addGet(Test::getTestUrl('8005') . '?seconds=1');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R0 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['0']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['0']['start_diff']);
+        // Assert R0 ends around 2.
+        $this->assertGreaterThanOrEqual(1.8, $request_stats['0']['stop_diff']);
+        $this->assertLessThanOrEqual(2.5, $request_stats['0']['stop_diff']);
+
+        // Assert R1 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['1']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['1']['start_diff']);
+        // Assert R1 ends around 9.
+        $this->assertGreaterThanOrEqual(8.8, $request_stats['1']['stop_diff']);
+        $this->assertLessThanOrEqual(9.5, $request_stats['1']['stop_diff']);
+
+        // Assert R2 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['2']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['2']['start_diff']);
+        // Assert R2 ends around 8.
+        $this->assertGreaterThanOrEqual(7.8, $request_stats['2']['stop_diff']);
+        $this->assertLessThanOrEqual(8.5 + 1, $request_stats['2']['stop_diff']);
+
+        // Assert R3 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['3']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['3']['start_diff']);
+        // Assert R3 ends around 11.
+        $this->assertGreaterThanOrEqual(10.8, $request_stats['3']['stop_diff']);
+        $this->assertLessThanOrEqual(11.5 + 1, $request_stats['3']['stop_diff']);
+
+        // Assert R4 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['4']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['4']['start_diff']);
+        // Assert R4 ends around 11.
+        $this->assertGreaterThanOrEqual(10.8, $request_stats['4']['stop_diff']);
+        $this->assertLessThanOrEqual(11.5 + 1, $request_stats['4']['stop_diff']);
+    }
+
+    public function testSetRateLimitPerSecond5()
+    {
+        //  R0--------------------------|
+        //  R1--------------------------|
+        //                      R2------|
+        //                      R3------|
+        //                              W-----------|
+        //                                          R4------|
+        //  0   1   2   3   4   5   6   7   8   9   10  11  12
+
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('2/5s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=7');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=7');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=2');
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=2');
+        $multi_curl->addGet(Test::getTestUrl('8005') . '?seconds=2');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R0 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['0']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['0']['start_diff']);
+        // Assert R0 ends around 7.
+        $this->assertGreaterThanOrEqual(6.8, $request_stats['0']['stop_diff']);
+        $this->assertLessThanOrEqual(7.5, $request_stats['0']['stop_diff']);
+
+        // Assert R1 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['1']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['1']['start_diff']);
+        // Assert R1 ends around 7.
+        $this->assertGreaterThanOrEqual(6.8, $request_stats['1']['stop_diff']);
+        $this->assertLessThanOrEqual(7.5, $request_stats['1']['stop_diff']);
+
+        // Assert R2 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['2']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['2']['start_diff']);
+        // Assert R2 ends around 7.
+        $this->assertGreaterThanOrEqual(6.8, $request_stats['2']['stop_diff']);
+        $this->assertLessThanOrEqual(7.5 + 1, $request_stats['2']['stop_diff']);
+
+        // Assert R3 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['3']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['3']['start_diff']);
+        // Assert R3 ends around 7.
+        $this->assertGreaterThanOrEqual(6.8, $request_stats['3']['stop_diff']);
+        $this->assertLessThanOrEqual(7.5 + 1, $request_stats['3']['stop_diff']);
+
+        // Assert R4 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['4']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['4']['start_diff']);
+        // Assert R4 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['4']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5, $request_stats['4']['stop_diff']);
+    }
+
+    public function testSetRateLimitPerSecond6()
+    {
+        //  R0--------------------------|
+        //  R1--------------------------|
+        //                      R2--------------|
+        //                      R3------|
+        //                                      W---|
+        //                                          R4------|
+        //  0   1   2   3   4   5   6   7   8   9   10  11  12
+
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('2/5s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=7');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=7');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=4');
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=2');
+        $multi_curl->addGet(Test::getTestUrl('8005') . '?seconds=2');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R0 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['0']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['0']['start_diff']);
+        // Assert R0 ends around 7.
+        $this->assertGreaterThanOrEqual(6.8, $request_stats['0']['stop_diff']);
+        $this->assertLessThanOrEqual(7.5, $request_stats['0']['stop_diff']);
+
+        // Assert R1 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['1']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['1']['start_diff']);
+        // Assert R1 ends around 7.
+        $this->assertGreaterThanOrEqual(6.8, $request_stats['1']['stop_diff']);
+        $this->assertLessThanOrEqual(7.5, $request_stats['1']['stop_diff']);
+
+        // Assert R2 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['2']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['2']['start_diff']);
+        // Assert R2 ends around 9.
+        $this->assertGreaterThanOrEqual(8.8, $request_stats['2']['stop_diff']);
+        $this->assertLessThanOrEqual(9.5 + 1, $request_stats['2']['stop_diff']);
+
+        // Assert R3 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['3']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['3']['start_diff']);
+        // Assert R3 ends around 7.
+        $this->assertGreaterThanOrEqual(6.8, $request_stats['3']['stop_diff']);
+        $this->assertLessThanOrEqual(7.5 + 1, $request_stats['3']['stop_diff']);
+
+        // Assert R4 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['4']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['4']['start_diff']);
+        // Assert R4 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['4']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5, $request_stats['4']['stop_diff']);
+    }
+
+    public function testSetRateLimitPerSecond7()
+    {
+        //  R0------|
+        //  R1----------------------------------------------|
+        //                      R2----------------------|
+        //                      R3----------------------|
+        //                                          R4------|
+        //                                          R5------|
+        //  0   1   2   3   4   5   6   7   8   9   10  11  12
+
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('2/5s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=2');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=12');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=6');
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=6');
+        $multi_curl->addGet(Test::getTestUrl('8005') . '?seconds=2');
+        $multi_curl->addGet(Test::getTestUrl('8006') . '?seconds=2');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R0 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['0']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['0']['start_diff']);
+        // Assert R0 ends around 2.
+        $this->assertGreaterThanOrEqual(1.8, $request_stats['0']['stop_diff']);
+        $this->assertLessThanOrEqual(2.5, $request_stats['0']['stop_diff']);
+
+        // Assert R1 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['1']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['1']['start_diff']);
+        // Assert R1 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['1']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5, $request_stats['1']['stop_diff']);
+
+        // Assert R2 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['2']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['2']['start_diff']);
+        // Assert R2 ends around 11.
+        $this->assertGreaterThanOrEqual(10.8, $request_stats['2']['stop_diff']);
+        $this->assertLessThanOrEqual(11.5 + 1, $request_stats['2']['stop_diff']);
+
+        // Assert R3 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['3']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['3']['start_diff']);
+        // Assert R3 ends around 11.
+        $this->assertGreaterThanOrEqual(10.8, $request_stats['3']['stop_diff']);
+        $this->assertLessThanOrEqual(11.5 + 1, $request_stats['3']['stop_diff']);
+
+        // Assert R4 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['4']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['4']['start_diff']);
+        // Assert R4 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['4']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5 + 1, $request_stats['4']['stop_diff']);
+
+        // Assert R5 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['5']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['5']['start_diff']);
+        // Assert R5 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['5']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5 + 1, $request_stats['5']['stop_diff']);
+    }
+
+    public function testSetRateLimitPerSecond8()
+    {
+        //  R0------------------------------|
+        //  R1----------------------------------------------|
+        //                      R2--------------------------|
+        //                      R3--------------|
+        //                                          R4--|
+        //                                          R5------|
+        //  0   1   2   3   4   5   6   7   8   9   10  11  12
+
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('2/5s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=8');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=12');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=7');
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=4');
+        $multi_curl->addGet(Test::getTestUrl('8005') . '?seconds=1');
+        $multi_curl->addGet(Test::getTestUrl('8006') . '?seconds=2');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R0 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['0']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['0']['start_diff']);
+        // Assert R0 ends around 8.
+        $this->assertGreaterThanOrEqual(7.8, $request_stats['0']['stop_diff']);
+        $this->assertLessThanOrEqual(8.5, $request_stats['0']['stop_diff']);
+
+        // Assert R1 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['1']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['1']['start_diff']);
+        // Assert R1 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['1']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5, $request_stats['1']['stop_diff']);
+
+        // Assert R2 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['2']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['2']['start_diff']);
+        // Assert R2 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['2']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5 + 1, $request_stats['2']['stop_diff']);
+
+        // Assert R3 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['3']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['3']['start_diff']);
+        // Assert R3 ends around 9.
+        $this->assertGreaterThanOrEqual(8.8, $request_stats['3']['stop_diff']);
+        $this->assertLessThanOrEqual(9.5 + 1, $request_stats['3']['stop_diff']);
+
+        // Assert R4 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['4']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['4']['start_diff']);
+        // Assert R4 ends around 11.
+        $this->assertGreaterThanOrEqual(10.8, $request_stats['4']['stop_diff']);
+        $this->assertLessThanOrEqual(11.5, $request_stats['4']['stop_diff']);
+
+        // Assert R5 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['5']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['5']['start_diff']);
+        // Assert R5 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['5']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5, $request_stats['5']['stop_diff']);
+    }
+
+    public function testSetRateLimitPerSecond9()
+    {
+        //  R0----------------------------------------------|
+        //  R1----------------------------------------------|
+        //                      R2--------------------------|
+        //                      R3--------------------------|
+        //                                          R4------|
+        //                                          R5------|
+        //  0   1   2   3   4   5   6   7   8   9   10  11  12
+
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('2/5s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=12');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=12');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=7');
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=7');
+        $multi_curl->addGet(Test::getTestUrl('8005') . '?seconds=2');
+        $multi_curl->addGet(Test::getTestUrl('8006') . '?seconds=2');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R0 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['0']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['0']['start_diff']);
+        // Assert R0 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['0']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5, $request_stats['0']['stop_diff']);
+
+        // Assert R1 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['1']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['1']['start_diff']);
+        // Assert R1 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['1']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5, $request_stats['1']['stop_diff']);
+
+        // Assert R2 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['2']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['2']['start_diff']);
+        // Assert R2 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['2']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5 + 1, $request_stats['2']['stop_diff']);
+
+        // Assert R3 starts around 5 and not before.
+        $this->assertGreaterThanOrEqual(5, $request_stats['3']['start_diff']);
+        $this->assertLessThanOrEqual(5.5, $request_stats['3']['start_diff']);
+        // Assert R3 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['3']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5 + 1, $request_stats['3']['stop_diff']);
+
+        // Assert R4 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['4']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['4']['start_diff']);
+        // Assert R4 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['4']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5 + 1, $request_stats['4']['stop_diff']);
+
+        // Assert R5 starts around 10 and not before.
+        $this->assertGreaterThanOrEqual(10, $request_stats['5']['start_diff']);
+        $this->assertLessThanOrEqual(10.5, $request_stats['5']['start_diff']);
+        // Assert R5 ends around 12.
+        $this->assertGreaterThanOrEqual(11.8, $request_stats['5']['stop_diff']);
+        $this->assertLessThanOrEqual(12.5 + 1, $request_stats['5']['stop_diff']);
+    }
+
+    public function testSetRateLimitPerSecondOnePerSecond()
+    {
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setRateLimit('1/1s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8001'));
+        $multi_curl->addGet(Test::getTestUrl('8002'));
+        $multi_curl->addGet(Test::getTestUrl('8003'));
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R0 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['0']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['0']['start_diff']);
+
+        // Assert R1 starts around 1 and not before.
+        $this->assertGreaterThanOrEqual(1, $request_stats['1']['start_diff']);
+        $this->assertLessThanOrEqual(1.5, $request_stats['1']['start_diff']);
+
+        // Assert R2 starts around 2 and not before.
+        $this->assertGreaterThanOrEqual(2, $request_stats['2']['start_diff']);
+        $this->assertLessThanOrEqual(2.5, $request_stats['2']['start_diff']);
+    }
+
+    public function testSetRateLimitTenPerThirtySecond()
+    {
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('5/30s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=15');
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=25');
+        $multi_curl->addGet(Test::getTestUrl('8006') . '?seconds=35');
+        $multi_curl->addGet(Test::getTestUrl('8005') . '?seconds=45');
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=20');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=10');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R5 starts around 30 and not before.
+        $this->assertGreaterThanOrEqual(30, $request_stats['5']['start_diff']);
+        $this->assertLessThanOrEqual(30.5, $request_stats['5']['start_diff']);
+    }
+
+    public function testSetRateLimitOnePerOneMinute()
+    {
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('1/1m');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=30');
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=70');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=10');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R1 starts around 60 and not before.
+        $this->assertGreaterThanOrEqual(60, $request_stats['1']['start_diff']);
+        $this->assertLessThanOrEqual(60.5, $request_stats['1']['start_diff']);
+        // Assert R2 starts around 120 and not before.
+        $this->assertGreaterThanOrEqual(120, $request_stats['2']['start_diff']);
+        $this->assertLessThanOrEqual(120.5, $request_stats['2']['start_diff']);
+    }
+
+    public function testSetRateLimitThreePerOneMinute()
+    {
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('3/1m');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=20');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=65');
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=45');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=10');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R3 starts around 60 and not before.
+        $this->assertGreaterThanOrEqual(60, $request_stats['3']['start_diff']);
+        $this->assertLessThanOrEqual(60.5, $request_stats['3']['start_diff']);
+    }
+
+    public function testSetRateLimitThreePerSixtyFiveSeconds()
+    {
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('3/65s');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        $multi_curl->addGet(Test::getTestUrl('8001') . '?seconds=5');
+        $multi_curl->addGet(Test::getTestUrl('8002') . '?seconds=5');
+        $multi_curl->addGet(Test::getTestUrl('8003') . '?seconds=5');
+        $multi_curl->addGet(Test::getTestUrl('8004') . '?seconds=5');
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // Assert R3 starts around 65 and not before.
+        $this->assertGreaterThanOrEqual(65, $request_stats['3']['start_diff']);
+        $this->assertLessThanOrEqual(65.5, $request_stats['3']['start_diff']);
+    }
+
+    public function testSetRateLimitTenPerTwoMinutes()
+    {
+        $request_stats = array(
+            'start' => '',
+            'stop' => '',
+        );
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'timeout');
+        $multi_curl->setRateLimit('10/2m');
+        $multi_curl->beforeSend(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id] = array();
+            $request_stats[$instance->id]['start'] = microtime(true);
+        });
+        $multi_curl->complete(function ($instance) use (&$request_stats) {
+            $request_stats[$instance->id]['stop'] = microtime(true);
+        });
+
+        for ($i = 0; $i <= 30; $i++) {
+            $multi_curl->addGet(Test::TEST_URL . '?seconds=1');
+        }
+
+        $request_stats['start'] = microtime(true);
+        $multi_curl->start();
+        $request_stats['stop'] = microtime(true);
+
+        foreach ($request_stats as $key => &$value) {
+            if (is_int($key)) {
+                $value['start_diff'] = $value['start'] - $request_stats['start'];
+                $value['stop_diff'] = $value['stop'] - $request_stats['start'];
+            }
+        }
+
+        // 0-9 starts >= 0.
+        // 10-19 starts >= 120.
+        // 20-29 starts >= 240.
+        // 30-39 starts >= 360.
+
+        // Assert R0 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['0']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['0']['start_diff']);
+        // Assert R9 starts around 0 and not before.
+        $this->assertGreaterThanOrEqual(0, $request_stats['9']['start_diff']);
+        $this->assertLessThanOrEqual(0.5, $request_stats['9']['start_diff']);
+
+        // Assert R10 starts around 120 and not before.
+        $this->assertGreaterThanOrEqual(120, $request_stats['10']['start_diff']);
+        $this->assertLessThanOrEqual(120.5, $request_stats['10']['start_diff']);
+        // Assert R19 starts around 120 and not before.
+        $this->assertGreaterThanOrEqual(120, $request_stats['19']['start_diff']);
+        $this->assertLessThanOrEqual(120.5, $request_stats['19']['start_diff']);
+
+        // Assert R20 starts around 240. Allow for some drift.
+        $this->assertGreaterThanOrEqual(239, $request_stats['20']['start_diff']);
+        $this->assertLessThanOrEqual(241, $request_stats['20']['start_diff']);
+        // Assert R29 starts around 240. Allow for some drift.
+        $this->assertGreaterThanOrEqual(239, $request_stats['29']['start_diff']);
+        $this->assertLessThanOrEqual(241, $request_stats['29']['start_diff']);
+
+        // Assert R30 starts around 360. Allow for some drift.
+        $this->assertGreaterThanOrEqual(359, $request_stats['30']['start_diff']);//
+        $this->assertLessThanOrEqual(361, $request_stats['30']['start_diff']);
+    }
 }
diff --git a/tests/PHPCurlClass/server.php b/tests/PHPCurlClass/server.php
index fc4ab7b..aec56d2 100644
--- a/tests/PHPCurlClass/server.php
+++ b/tests/PHPCurlClass/server.php
@@ -1,4 +1,6 @@
 <?php
+$server_start = microtime(true);
+
 // Prevent direct access unless testing.
 if (getenv('PHP_CURL_CLASS_TEST_MODE_ENABLED') !== 'yes' &&
     @$_SERVER['PHP_CURL_CLASS_TEST_MODE_ENABLED'] !== 'yes') {
@@ -291,8 +293,12 @@ if ($test === 'http_basic_auth') {
             break;
         }
     }
+
     echo '",' . "\n";
-    echo '  "elapsed_seconds": "' . $elapsed . '"' . "\n";
+    echo '  "elapsed_seconds": "' . $elapsed . '",' . "\n";
+    echo '  "server_port": "' . ((int)$_SERVER['SERVER_PORT']) . '",' . "\n";
+    echo '  "server_start": "' . $server_start . '",' . "\n";
+    echo '  "server_stop": "' . microtime(true) . '"' . "\n";
     echo '}' . "\n";
     exit;
 } elseif ($test === 'error_message') {
diff --git a/tests/before_script.sh b/tests/before_script.sh
index 0c4fba6..726d24a 100755
--- a/tests/before_script.sh
+++ b/tests/before_script.sh
@@ -85,6 +85,14 @@ phpunit_v7_5_shim() {
     remove_expectWarning
 }
 
+start_php_servers() {
+    for i in $(seq 0 6); do
+        port=8000
+        (( port += $i ))
+        php -S "127.0.0.1:${port}" -t tests/PHPCurlClass/ &
+    done
+}
+
 set -x
 echo "TRAVIS_PHP_VERSION: ${TRAVIS_PHP_VERSION}"
 php -r "var_dump(phpversion());"
@@ -160,16 +168,16 @@ elif [[ "${TRAVIS_PHP_VERSION}" == "5.6" ]]; then
     phpunit_shim
 elif [[ "${TRAVIS_PHP_VERSION}" == "7.0" ]]; then
     phpunit_v6_5_shim
-    php -S 127.0.0.1:8000 -t tests/PHPCurlClass/ &
+    start_php_servers
 elif [[ "${TRAVIS_PHP_VERSION}" == "7.1" ]]; then
     phpunit_v7_5_shim
-    php -S 127.0.0.1:8000 -t tests/PHPCurlClass/ &
+    start_php_servers
 elif [[ "${TRAVIS_PHP_VERSION}" == "7.2" ]]; then
-    php -S 127.0.0.1:8000 -t tests/PHPCurlClass/ &
+    start_php_servers
 elif [[ "${TRAVIS_PHP_VERSION}" == "7.3" ]]; then
-    php -S 127.0.0.1:8000 -t tests/PHPCurlClass/ &
+    start_php_servers
 elif [[ "${TRAVIS_PHP_VERSION}" == "7.4" ]]; then
-    php -S 127.0.0.1:8000 -t tests/PHPCurlClass/ &
+    start_php_servers
 elif [[ "${TRAVIS_PHP_VERSION}" == "hhvm" || "${TRAVIS_PHP_VERSION}" == "hhvm-nightly" ]]; then
     curl "https://nginx.org/keys/nginx_signing.key" | sudo apt-key add -
     echo "deb https://nginx.org/packages/mainline/ubuntu/ trusty nginx" | sudo tee -a /etc/apt/sources.list
@@ -210,5 +218,5 @@ EOF
         composer require phpunit/phpunit:5.7.*
     fi
 elif [[ "${TRAVIS_PHP_VERSION}" == "nightly" ]]; then
-    php -S 127.0.0.1:8000 -t tests/PHPCurlClass/ &
+    start_php_servers
 fi
diff --git a/tests/run.sh b/tests/run.sh
index 7108210..b81c74a 100755
--- a/tests/run.sh
+++ b/tests/run.sh
@@ -6,9 +6,22 @@ set -x
 # Let test server know we should allow testing.
 export PHP_CURL_CLASS_TEST_MODE_ENABLED="yes"
 
-# Start test server.
-php -S 127.0.0.1:8000 -t PHPCurlClass/ &> /dev/null &
-pid="${!}"
+# Let test server know this is a local test.
+export PHP_CURL_CLASS_LOCAL_TEST="yes"
+
+# Start test servers. Run servers on different ports to allow simultaneous
+# requests without blocking.
+server_count=7
+declare -A pids
+for i in $(seq 0 $(("${server_count}" - 1))); do
+    port=8000
+    (( port += $i ))
+
+    php -S "127.0.0.1:${port}" -t PHPCurlClass/ &> /dev/null &
+    pid="${!}"
+
+    pids["${i}"]="${pid}"
+done
 
 # Determine which phpunit to use.
 if [[ -f "../vendor/phpunit/phpunit/phpunit" ]]; then
@@ -24,4 +37,8 @@ extra_args="${@}"
     --debug \
     --verbose \
     ${extra_args}
-kill "${pid}"
+
+# Stop test servers.
+for pid in "${pids[@]}"; do
+  kill "${pid}"
+done
diff --git a/tests/start_servers.sh b/tests/start_servers.sh
deleted file mode 100755
index e4bb660..0000000
--- a/tests/start_servers.sh
+++ /dev/null
@@ -1,24 +0,0 @@
-screen_name="my_screen"
-server_count=5
-# screen_binary="screen"
-screen_binary="byobu-screen"
-
-# Start screen in detached mode with a session name.
-screen -S "${screen_name}" -t "master" -d -m
-
-# Wait for screen to be ready before opening new sessions.
-sleep 1
-
-# Create tabs and send commands to each.
-for i in $(seq 1 "${server_count}"); do
-    # Create tab.
-    screen -S "${screen_name}" -X "screen" -t "my_screen_${i}"
-
-    # Start development server in tab.
-    port=8000
-    (( port += $i ))
-    command="php -S 127.0.0.1:${port} -t PHPCurlClass/"
-    screen -S "${screen_name}" -p "my_screen_${i}" -X stuff "${command}"$'\n'
-done
-
-screen -r "${screen_name}"