错误信息：PHP Fatal error:  Swoole\Http\Server::start(): failed to start server. Error: swServer_start_check: onTask is null in /var/www/html/swoft/websocket.php on line 28

<?php

$server = new swoole_websocket_server("0.0.0.0", 8080,SWOOLE_PROCESS,SWOOLE_SOCK_TCP | SWOOLE_SSL);

//$server = new swoole_websocket_server("0.0.0.0", 8080);

   //设置ws服务参数

 $server->set(array(

 'worker_num' => 2,

'ssl_key_file' => '/etc/httpd/conf/3_www.hongtoo.cn.key',

'ssl_cert_file' => '/etc/httpd/conf/2_www.hongtoo.cn.crt',

'task_worker_num' => 2,

'heartbeat_check_interval' => 60,   //心跳测试 每隔多少秒检测一次

'heartbeat_idle_time' => 600   //TCP连接的最大闲置时间

 )); 

$server->on('open', function (swoole_websocket_server $server, $request) {

    echo "server: handshake success with fd{$request->fd}\n";

});

$server->on('message', function (swoole_websocket_server $server, $frame) {

    echo "receive from {$frame->fd}:{$frame->data},opcode:{$frame->opcode},fin:{$frame->finish}\n";

    $server->push($frame->fd, "this is server");

});

$server->on('close', function ($ser, $fd) {

    echo "client {$fd} closed\n";

});

$server->start();