code: protected function _call($fieldData, $controller, $method, $passing, $arguments)
      {
          // If data to be passed to the method isn't an array, put it in array format.
          $fieldData = array($fieldData, $arguments);
              
          // Call custom controller->method and pass the data to it.
          $result = call_user_func_array(array($controller, $method), $fieldData);
  
          // If the returned result meets expections, pass test (return false) otherwise return message.
          return $result == $passing ? true : false;
      }
docs: This handles when a user wants to call a custom validation method in some other class.
  A common example would be checking if a username is already taken in a database
  when using a user register form.
  
  $fieldData is the actual data.
  $controller is the controller that needs to be invoked for this custom check.
  $method is the method that needs to be called.
  $passing defines what the method should return to pass the test. Two Examples below:
  $user->nameExists($fieldData) you would want to return FALSE. So set $passing = false.
  $user->nameAvailable($fieldData) you would want to return TRUE. So set $passing = true.