Updates PHPUnit version.

Author: aidan-casey

.github/workflows/isolated-tests.yml

@@ -59,7 +59,7 @@ jobs:
           coverage: pcov
 
       - name: Install PHPUnit
-        run: composer global require phpunit/phpunit
+        run: composer global require phpunit/phpunit:^11.3
 
       - name: Setup problem matchers
         run: |