diff --git a/src/Curl/ArrayUtil.php b/src/Curl/ArrayUtil.php
index 61834de..00277b2 100644
--- a/src/Curl/ArrayUtil.php
+++ b/src/Curl/ArrayUtil.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Curl;
 
diff --git a/src/Curl/CaseInsensitiveArray.php b/src/Curl/CaseInsensitiveArray.php
index 2592265..5ae1db0 100644
--- a/src/Curl/CaseInsensitiveArray.php
+++ b/src/Curl/CaseInsensitiveArray.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Curl;
 
diff --git a/src/Curl/Curl.php b/src/Curl/Curl.php
index 980bef5..334dfd7 100644
--- a/src/Curl/Curl.php
+++ b/src/Curl/Curl.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Curl;
 
diff --git a/src/Curl/Decoder.php b/src/Curl/Decoder.php
index 3170811..603f4c3 100644
--- a/src/Curl/Decoder.php
+++ b/src/Curl/Decoder.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Curl;
 
diff --git a/src/Curl/Encoder.php b/src/Curl/Encoder.php
index bce8ed6..d999521 100644
--- a/src/Curl/Encoder.php
+++ b/src/Curl/Encoder.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Curl;
 
diff --git a/src/Curl/MultiCurl.php b/src/Curl/MultiCurl.php
index 79ec568..13b9f8c 100755
--- a/src/Curl/MultiCurl.php
+++ b/src/Curl/MultiCurl.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Curl;
 
diff --git a/src/Curl/StringUtil.php b/src/Curl/StringUtil.php
index e5ca0e9..86341f9 100644
--- a/src/Curl/StringUtil.php
+++ b/src/Curl/StringUtil.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Curl;
 
diff --git a/src/Curl/Url.php b/src/Curl/Url.php
index b37debe..d811fc9 100644
--- a/src/Curl/Url.php
+++ b/src/Curl/Url.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Curl;
 
diff --git a/tests/PHPCurlClass/ArrayUtilTest.php b/tests/PHPCurlClass/ArrayUtilTest.php
index 0f15395..95dc6a8 100644
--- a/tests/PHPCurlClass/ArrayUtilTest.php
+++ b/tests/PHPCurlClass/ArrayUtilTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace CurlTest;
 
diff --git a/tests/PHPCurlClass/ContentRangeServer.php b/tests/PHPCurlClass/ContentRangeServer.php
index 8e6e6d5..c691d73 100644
--- a/tests/PHPCurlClass/ContentRangeServer.php
+++ b/tests/PHPCurlClass/ContentRangeServer.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace ContentRangeServer;
 
diff --git a/tests/PHPCurlClass/Helper.php b/tests/PHPCurlClass/Helper.php
index 9ac0d62..79ded31 100644
--- a/tests/PHPCurlClass/Helper.php
+++ b/tests/PHPCurlClass/Helper.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Helper;
 
diff --git a/tests/PHPCurlClass/PHPCurlClassTest.php b/tests/PHPCurlClass/PHPCurlClassTest.php
index 4a30517..5c994ca 100644
--- a/tests/PHPCurlClass/PHPCurlClassTest.php
+++ b/tests/PHPCurlClass/PHPCurlClassTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace CurlTest;
 
diff --git a/tests/PHPCurlClass/PHPMultiCurlClassTest.php b/tests/PHPCurlClass/PHPMultiCurlClassTest.php
index 8e60d2d..92bf42c 100644
--- a/tests/PHPCurlClass/PHPMultiCurlClassTest.php
+++ b/tests/PHPCurlClass/PHPMultiCurlClassTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace CurlTest;
 
diff --git a/tests/PHPCurlClass/RangeHeader.php b/tests/PHPCurlClass/RangeHeader.php
index 380d2c5..31d153f 100644
--- a/tests/PHPCurlClass/RangeHeader.php
+++ b/tests/PHPCurlClass/RangeHeader.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace RangeHeader;
 
diff --git a/tests/PHPCurlClass/UrlTest.php b/tests/PHPCurlClass/UrlTest.php
index 8d3effb..e8fa8fc 100644
--- a/tests/PHPCurlClass/UrlTest.php
+++ b/tests/PHPCurlClass/UrlTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace CurlTest;
 
diff --git a/tests/PHPCurlClass/User.php b/tests/PHPCurlClass/User.php
index 69fd468..409ca62 100644
--- a/tests/PHPCurlClass/User.php
+++ b/tests/PHPCurlClass/User.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Helper;
 
diff --git a/tests/PHPCurlClass/server.php b/tests/PHPCurlClass/server.php
index 29882f4..2f75d20 100644
--- a/tests/PHPCurlClass/server.php
+++ b/tests/PHPCurlClass/server.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 $server_start = microtime(true);
 
 // Prevent direct access unless testing.