function getdownurl($downurl, $extime = "3600", $serverid = 1) {
    if (empty($downurl)) {
        return -1;
    }
    //需要在配置文件中设置（多组）下载服务器的地址
    $servers = $GLOBALS['downservers'];
    if (!array_key_exists($serverid, $servers)) {
        return -2;
    }
    $secret = 'heL_lo';
    $expire = time() + $extime;
    $md5 = base64_encode(md5($secret . $downurl . $expire, true));
    $md5 = strtr($md5, '+/', '-_');
    $md5 = str_replace('=', '', $md5);
    $down_url = "http://" . $servers[$serverid]['domain'] . $downurl . "?st=$md5&e=$expire";
    return $down_url;
}