public static function uuid($key = null, $prefix = '')
    {
        $key = ($key == null) ? uniqid(rand()) : $key;
        $chars = md5($key);
        $uuid  = substr($chars,0,8) . '-';
        $uuid .= substr($chars,8,4) . '-';
        $uuid .= substr($chars,12,4) . '-';
        $uuid .= substr($chars,16,4) . '-';
        $uuid .= substr($chars,20,12);

        return $prefix . $uuid;
    }