diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 1105f82..95815f1 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -11,6 +11,7 @@ jobs:
     strategy:
       matrix:
         php:
+          - 8.3
           - 8.2
           - 8.1
           - 8.0
@@ -24,7 +25,7 @@ jobs:
           - 5.4
           - 5.3
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php }}
diff --git a/composer.json b/composer.json
index 3dceb31..cfb603d 100644
--- a/composer.json
+++ b/composer.json
@@ -31,7 +31,7 @@
         "react/promise": "^3.0 || ^2.8 || ^1.2.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.5 || ^5.7 || ^4.8.35"
+        "phpunit/phpunit": "^9.6 || ^5.7 || ^4.8.35"
     },
     "autoload": {
         "files": [
@@ -39,6 +39,8 @@
         ]
     },
     "autoload-dev": {
-        "psr-4": { "React\\Tests\\Async\\": "tests/" }
+        "psr-4": {
+            "React\\Tests\\Async\\": "tests/"
+        }
     }
 }
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 7a9577e..ac542e7 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,8 +1,8 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<!-- PHPUnit configuration file with new format for PHPUnit 9.5+ -->
+<!-- PHPUnit configuration file with new format for PHPUnit 9.6+ -->
 <phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.5/phpunit.xsd"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.6/phpunit.xsd"
          bootstrap="vendor/autoload.php"
          cacheResult="false"
          colors="true"
@@ -20,7 +20,7 @@
     <php>
         <ini name="error_reporting" value="-1" />
         <!-- Evaluate assertions, requires running with "php -d zend.assertions=1 vendor/bin/phpunit" -->
-        <!-- <ini name="zend.assertions=1" value="1" /> -->
+        <!-- <ini name="zend.assertions" value="1" /> -->
         <ini name="assert.active" value="1" />
         <ini name="assert.exception" value="1" />
         <ini name="assert.bail" value="0" />
diff --git a/phpunit.xml.legacy b/phpunit.xml.legacy
index 9799323..8916116 100644
--- a/phpunit.xml.legacy
+++ b/phpunit.xml.legacy
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<!-- PHPUnit configuration file with old format before PHPUnit 9 -->
+<!-- PHPUnit configuration file with old format for legacy PHPUnit -->
 <phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/4.8/phpunit.xsd"
          bootstrap="vendor/autoload.php"
@@ -18,7 +18,7 @@
     <php>
         <ini name="error_reporting" value="-1" />
         <!-- Evaluate assertions, requires running with "php -d zend.assertions=1 vendor/bin/phpunit" -->
-        <!-- <ini name="zend.assertions=1" value="1" /> -->
+        <!-- <ini name="zend.assertions" value="1" /> -->
         <ini name="assert.active" value="1" />
         <ini name="assert.exception" value="1" />
         <ini name="assert.bail" value="0" />
diff --git a/src/functions_include.php b/src/functions_include.php
index 92a7439..05c78fa 100644
--- a/src/functions_include.php
+++ b/src/functions_include.php
@@ -3,6 +3,7 @@
 namespace React\Async;
 
 // @codeCoverageIgnoreStart
-if (!function_exists(__NAMESPACE__ . '\\parallel')) {
+if (!\function_exists(__NAMESPACE__ . '\\parallel')) {
     require __DIR__ . '/functions.php';
 }
+// @codeCoverageIgnoreEnd