Check for PHP startup warnings

.github/workflows/test_php_ext.yml

@@ -60,5 +60,14 @@ jobs:
           MAKE="make -j$(nproc)" pecl install $GITHUB_WORKSPACE/protobuf-*.tgz
       - name: Enable extension
         run: docker-php-ext-enable protobuf
+      - name: Check for PHP startup warnings
+        run: |
+          php -d display_errors=stderr -d display_startup_errors=1 -d error_reporting=-1 -r ';' 2>/tmp/protobuf-warnings
+          if [ -s /tmp/protobuf-warnings ]; then
+            echo 'The PHP extension was successfully installed, but PHP raised these warnings:' >&2
+            cat /tmp/protobuf-warnings >&2
+            exit 1
+          fi
+          echo "PHP didn't raise any warnings at startup."
       - name: Inspect extension
         run: php --ri protobuf