diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
index 7016929..2c79089 100644
--- a/.github/workflows/php.yml
+++ b/.github/workflows/php.yml
@@ -18,7 +18,7 @@ jobs:
         php: ['8.4', '8.5']
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 
     - name: Setup PHP
       uses: shivammathur/setup-php@v2
@@ -32,7 +32,7 @@ jobs:
 
     - name: Cache Composer packages
       id: composer-cache
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       with:
         path: vendor
         key: ${{ runner.os }}-php-${{ matrix.php }}-${{ hashFiles('**/composer.lock') }}
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index c88259c..4253460 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.5/phpunit.xsd" colors="true" bootstrap="src/autoload.php" cacheDirectory=".phpunit.cache">
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/13.0/phpunit.xsd" colors="true" bootstrap="src/autoload.php" cacheDirectory=".phpunit.cache">
   <coverage>
     <report>
       <clover outputFile="build/logs/clover.xml"/>
diff --git a/src/ReCaptcha/ReCaptcha.php b/src/ReCaptcha/ReCaptcha.php
index c099d39..4a000cb 100644
--- a/src/ReCaptcha/ReCaptcha.php
+++ b/src/ReCaptcha/ReCaptcha.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
diff --git a/src/ReCaptcha/RequestMethod.php b/src/ReCaptcha/RequestMethod.php
index 5b4211f..f39234a 100644
--- a/src/ReCaptcha/RequestMethod.php
+++ b/src/ReCaptcha/RequestMethod.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
diff --git a/src/ReCaptcha/RequestMethod/CurlPost.php b/src/ReCaptcha/RequestMethod/CurlPost.php
index 55bbfd0..5ff831a 100644
--- a/src/ReCaptcha/RequestMethod/CurlPost.php
+++ b/src/ReCaptcha/RequestMethod/CurlPost.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
@@ -89,12 +91,16 @@ public function submit(RequestParameters $params): string
         ];
         curl_setopt_array($handle, $options);
 
-        $response = curl_exec($handle);
+        try {
+            $response = curl_exec($handle);
 
-        if (is_string($response)) {
-            return $response;
-        }
+            if (is_string($response)) {
+                return $response;
+            }
 
-        return '{"success": false, "error-codes": ["'.ReCaptcha::E_CONNECTION_FAILED.'"]}';
+            return '{"success": false, "error-codes": ["'.ReCaptcha::E_CONNECTION_FAILED.'"]}';
+        } finally {
+            curl_close($handle);
+        }
     }
 }
diff --git a/src/ReCaptcha/RequestMethod/Post.php b/src/ReCaptcha/RequestMethod/Post.php
index 37937c2..77ff691 100644
--- a/src/ReCaptcha/RequestMethod/Post.php
+++ b/src/ReCaptcha/RequestMethod/Post.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
diff --git a/src/ReCaptcha/RequestMethod/SocketPost.php b/src/ReCaptcha/RequestMethod/SocketPost.php
index e0a6a81..1dab4db 100644
--- a/src/ReCaptcha/RequestMethod/SocketPost.php
+++ b/src/ReCaptcha/RequestMethod/SocketPost.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
diff --git a/src/ReCaptcha/RequestParameters.php b/src/ReCaptcha/RequestParameters.php
index 742713a..2614b41 100644
--- a/src/ReCaptcha/RequestParameters.php
+++ b/src/ReCaptcha/RequestParameters.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
diff --git a/src/ReCaptcha/Response.php b/src/ReCaptcha/Response.php
index efa2e6a..852439b 100644
--- a/src/ReCaptcha/Response.php
+++ b/src/ReCaptcha/Response.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
diff --git a/src/autoload.php b/src/autoload.php
index 1866970..b4b7c53 100644
--- a/src/autoload.php
+++ b/src/autoload.php
@@ -35,7 +35,7 @@
  */
 
 spl_autoload_register(function ($class) {
-    if ('ReCaptcha\\' !== substr($class, 0, 10)) {
+    if (!str_starts_with($class, 'ReCaptcha\\')) {
         /* If the class does not lie under the "ReCaptcha" namespace,
          * then we can exit immediately.
          */
diff --git a/tests/ReCaptcha/ReCaptchaTest.php b/tests/ReCaptcha/ReCaptchaTest.php
index 809a514..f7f85e8 100644
--- a/tests/ReCaptcha/ReCaptchaTest.php
+++ b/tests/ReCaptcha/ReCaptchaTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
@@ -90,6 +92,7 @@ public static function invalidSecretProvider(): array
             [null],
             [new \stdClass()],
             [[]],
+            [0],
         ];
     }
 
@@ -109,7 +112,6 @@ public static function emptySecretProvider(): array
     {
         return [
             [''],
-            [0],
         ];
     }
 
diff --git a/tests/ReCaptcha/RequestMethod/CurlPostTest.php b/tests/ReCaptcha/RequestMethod/CurlPostTest.php
index e719fbc..88e4d39 100644
--- a/tests/ReCaptcha/RequestMethod/CurlPostTest.php
+++ b/tests/ReCaptcha/RequestMethod/CurlPostTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
@@ -86,6 +88,14 @@ function curl_exec(\stdClass $ch): bool|string
     return CurlPostGlobalState::$execResponse;
 }
 
+/**
+ * Mock curl_close in the ReCaptcha\RequestMethod namespace.
+ */
+function curl_close(\stdClass $ch): void
+{
+    // no-op mock
+}
+
 /**
  * @internal
  *
diff --git a/tests/ReCaptcha/RequestMethod/PostTest.php b/tests/ReCaptcha/RequestMethod/PostTest.php
index 16c5df9..e249a8a 100644
--- a/tests/ReCaptcha/RequestMethod/PostTest.php
+++ b/tests/ReCaptcha/RequestMethod/PostTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
diff --git a/tests/ReCaptcha/RequestMethod/SocketPostTest.php b/tests/ReCaptcha/RequestMethod/SocketPostTest.php
index c7ecb14..cf1107e 100644
--- a/tests/ReCaptcha/RequestMethod/SocketPostTest.php
+++ b/tests/ReCaptcha/RequestMethod/SocketPostTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
diff --git a/tests/ReCaptcha/RequestParametersTest.php b/tests/ReCaptcha/RequestParametersTest.php
index 3fdb4f5..8b5bcdc 100644
--- a/tests/ReCaptcha/RequestParametersTest.php
+++ b/tests/ReCaptcha/RequestParametersTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *
diff --git a/tests/ReCaptcha/ResponseTest.php b/tests/ReCaptcha/ResponseTest.php
index 2f6e0f5..bc8b21b 100644
--- a/tests/ReCaptcha/ResponseTest.php
+++ b/tests/ReCaptcha/ResponseTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This is a PHP library that handles calling reCAPTCHA.
  *