public static function validates($attr, $validation) 
    {
        $cls = get_called_class();

        if (!array_key_exists($cls, self::$_validations)) {
            self::$_validations[$cls] = array();
        }

        if (!array_key_exists($attr, self::$_validations[$cls])) {
            self::$_validations[$cls][$attr] = array();
        }

        array_push(self::$_validations[$cls][$attr], $validation);
    }