使此算法更有效

我是一个php noob，需要一些帮助。我建立了一个推送服务，可以向iphone发送推送消息。它在评论中表示，将多条消息组合成一个数据包更有效。此外，现在我打电话删除每一条信息。如何将多条消息组合成一个包？

感谢

function start()
{
    writeToLog('Connecting to ' . $this->server);
    if (!$this->connectToAPNS())
        exit;

    while (true)
    {
        // Do at most 20 messages at a time. Note: we send each message in
        // a separate packet to APNS. It would be more efficient if we 
        // combined several messages into one packet, but this script isn't
        // smart enough to do that. ;-)
        $stmt = $this->pdo->prepare('SELECT * FROM push_queue WHERE time_sent IS NULL LIMIT 20');
        $stmt->execute();
        $messages = $stmt->fetchAll(PDO::FETCH_OBJ);
        foreach ($messages as $message)
        {
            if ($this->sendNotification($message->message_id, $message->device_token, $message->payload))
            {
                //$stmt = $this->pdo->prepare('UPDATE push_queue SET time_sent = NOW() WHERE message_id = ?');
                //$stmt->execute(array($message->message_id));
                $stmt = $this->pdo->prepare('DELETE FROM push_queue WHERE message_id = ?');
                $stmt->execute(array($message->message_id));
            }
            else  // failed to deliver
            {
                $this->reconnectToAPNS();
            }
        }
        unset($messages);           
        sleep(5);
    }
}