PHPUnit 5.7 is the last one to be compatible with PHP 5.6

Indeed, PHPUnit 6 requires at least PHP 7

Author: pH-7

composer.json

@@ -26,7 +26,7 @@
     "paragonie/random_compat": "^2.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "^9.5"
+    "phpunit/phpunit": "^5.7"
   },
   "autoload": {
     "psr-4": {