diff --git a/src/Curl/ArrayUtil.php b/src/Curl/ArrayUtil.php
index babd2b7..e618657 100644
--- a/src/Curl/ArrayUtil.php
+++ b/src/Curl/ArrayUtil.php
@@ -83,27 +83,36 @@ class ArrayUtil
             if (empty($array)) {
                 $return[$prefix] = '';
             } else {
+                $arrays_to_merge = [];
+
                 foreach ($array as $key => $value) {
                     if (is_scalar($value)) {
                         if ($prefix) {
-                            $return[$prefix . '[' . $key . ']'] = $value;
+                            $arrays_to_merge[] = [
+                                $prefix . '[' . $key . ']' => $value,
+                            ];
                         } else {
-                            $return[$key] = $value;
+                            $arrays_to_merge[] = [
+                                $key => $value,
+                            ];
                         }
                     } elseif ($value instanceof \CURLFile) {
-                        $return[$key] = $value;
+                        $arrays_to_merge[] = [
+                            $key => $value,
+                        ];
                     } elseif ($value instanceof \CURLStringFile) {
-                        $return[$key] = $value;
+                        $arrays_to_merge[] = [
+                            $key => $value,
+                        ];
                     } else {
-                        $return = array_merge(
-                            $return,
-                            self::arrayFlattenMultidim(
-                                $value,
-                                $prefix ? $prefix . '[' . $key . ']' : $key
-                            )
+                        $arrays_to_merge[] = self::arrayFlattenMultidim(
+                            $value,
+                            $prefix ? $prefix . '[' . $key . ']' : $key
                         );
                     }
                 }
+
+                $return = array_merge($return, ...$arrays_to_merge);
             }
         } elseif ($array === null) {
             $return[$prefix] = $array;
diff --git a/tests/PHPCurlClass/ArrayUtilTest.php b/tests/PHPCurlClass/ArrayUtilTest.php
index e501b2d..e2561a4 100644
--- a/tests/PHPCurlClass/ArrayUtilTest.php
+++ b/tests/PHPCurlClass/ArrayUtilTest.php
@@ -9,7 +9,7 @@ class ArrayUtilTest extends \PHPUnit\Framework\TestCase
 {
     public function testArrayAssociative()
     {
-        $this->assertTrue(\Curl\ArrayUtil::isArrayAssoc([
+        $this->assertTrue(ArrayUtil::isArrayAssoc([
             'foo' => 'wibble',
             'bar' => 'wubble',
             'baz' => 'wobble',
@@ -18,7 +18,7 @@ class ArrayUtilTest extends \PHPUnit\Framework\TestCase
 
     public function testArrayIndexed()
     {
-        $this->assertFalse(\Curl\ArrayUtil::isArrayAssoc([
+        $this->assertFalse(ArrayUtil::isArrayAssoc([
             'wibble',
             'wubble',
             'wobble',
@@ -30,4 +30,61 @@ class ArrayUtilTest extends \PHPUnit\Framework\TestCase
         $array = new CaseInsensitiveArray();
         $this->assertTrue(ArrayUtil::isArrayAssoc($array));
     }
+
+    public function testArrayFlattenMultidimArray()
+    {
+        $data = array(
+            'key-1' => 'value-1',
+            'key-2' => 'value-2',
+            'key-3' => [
+                'nested-key-1' => 'nested-value-1',
+                'nested-key-2' => 'nested-value-2',
+                'nested-key-3' => [
+                    'nested-more-key-1' => 'nested-more-value-1',
+                    'nested-more-key-2' => 'nested-more-value-2',
+                ],
+            ],
+        );
+
+        $this->assertEquals([
+            'key-1' => 'value-1',
+            'key-2' => 'value-2',
+            'key-3[nested-key-1]' => 'nested-value-1',
+            'key-3[nested-key-2]' => 'nested-value-2',
+            'key-3[nested-key-3][nested-more-key-1]' => 'nested-more-value-1',
+            'key-3[nested-key-3][nested-more-key-2]' => 'nested-more-value-2',
+        ], ArrayUtil::arrayFlattenMultidim($data));
+    }
+
+    public function testArrayFlattenMultidimOrdering()
+    {
+        $data = [
+            'foo' => 'bar',
+            'baz' => [
+                'qux' => [
+                ],
+                'wibble' => 'wobble',
+            ],
+        ];
+
+        $result = ArrayUtil::arrayFlattenMultidim($data);
+
+        // Avoid using assertEquals() as it isn't strict about ordering:
+        //   $this->assertEquals([
+        //       'foo' => 'bar',
+        //       'baz[qux]' => '',
+        //       'baz[wibble]' => 'wobble',
+        //   ], ArrayUtil::arrayFlattenMultidim($data));
+
+        $result_keys = array_keys($result);
+        $result_values = array_values($result);
+
+        $this->assertEquals('foo', $result_keys[0]);
+        $this->assertEquals('baz[qux]', $result_keys[1]);
+        $this->assertEquals('baz[wibble]', $result_keys[2]);
+
+        $this->assertEquals('bar', $result_values[0]);
+        $this->assertEquals('', $result_values[1]);
+        $this->assertEquals('wobble', $result_values[2]);
+    }
 }