public static function getRootPid($uid = null)
    {
        static $cache = array();
        $ret = null;

        if ($uid === null) {
            #################
            # Current root PID
            #################
            $rootline = self::getRootLine();
            if (!empty($rootline[0])) {
                $ret = $rootline[0]['uid'];
            }
        } else {
            #################
            # Other root PID
            #################
            if (!isset($cache[$uid])) {
                $cache[$uid] = null;
                $rootline    = self::getRootLine($uid);

                if (!empty($rootline[0])) {
                    $cache[$uid] = $rootline[0]['uid'];
                }
            }

            $ret = $cache[$uid];
        }

        return $ret;
    }