GUMP::add_filter PHP Method

    public static function add_filter($rule, $callback)
    {
        $method = 'filter_' . $rule;
        if (method_exists(__CLASS__, $method) || isset(self::$filter_methods[$rule])) {
            throw new Exception("Filter rule '{$rule}' already exists.");
        }
        self::$filter_methods[$rule] = $callback;
        return true;
    }