I want to write as less code as I can (it's only a proof of concept), so I will reuse the assertion framework or PHPUnit. As I've seen when studying Behat, we can use the assertion part as standalone functions. We only need to include vendor/phpunit/phpunit/PHPUnit/Framework/Assert/Functions.php file.
He includes the sample code showing his basic interface - a string calculation object being passed into a closure for evaluation by the PHPUnit assertion methods. He also includes an example of using it along with Mockery for creating a mock object and testing based on that.