Bump actions/checkout to v4

v2 runs on the deprecated Node.js 20 runtime; v4 uses Node 24 and
clears the GitHub Actions deprecation warning.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Cannonb4ll

.github/workflows/php_test.yml

@@ -16,7 +16,7 @@ jobs:
     name: PHP ${{ matrix.php }}
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2