system/classes/Validate.php
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 == $passing ? true : false;
    }
protected function _call($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;
    }
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.