使用阿里云 + workerman，H5 websocket每隔一段时间会收到大写的PING字符串是怎么回事？

<?php
    use Workerman\Worker;
    require_once './Workerman/Autoloader.php';

    $ws_worker = new Worker("websocket://0.0.0.0:1100");

    $ws_worker->count = 1;

    // 当收到客户端发来的数据后返回$data给客户端
    $ws_worker->onMessage = function($connection, $data) use ($ws_worker) {
        foreach ($ws_worker->connections as $conn) {
            $conn->send($data);
        }
    };

    // 运行worker
    Worker::runAll();