Checks source trees for fully qualified function calls and constants.
composer require --dev kelunik/fqn-check
vendor/bin/fqn-check /path/to/source
All not fully qualified function calls and constants will be listed and the exit code will be 1 if any such function call is found, 0 otherwise.
You might also want to check out friendsofphp/php-cs-fixer with its native_function_invocation option.