public static function getUserIdSalt()
    {
        $salt = Option::get(self::OPTION_USERID_SALT);
        if (empty($salt)) {
            $salt = Common::getRandomString($len = 40, $alphabet = "abcdefghijklmnoprstuvwxyzABCDEFGHIJKLMNOPRSTUVWXYZ0123456789_-$");
            Option::set(self::OPTION_USERID_SALT, $salt, 1);
        }
        return $salt;
    }