Bluz\Validator\Rule\Callback::validate PHP Метод

    public function validate($input) : bool
    {
        return (bool) call_user_func($this->callback, $input);
    }

 public function testCallbackValidatorShouldAcceptFunctionNamesAsString()
 {
     $v = new Callback('is_string');
     $this->assertTrue($v->validate('test'));
 }