$db未定义

public static function onWorkerStart($worker)
   {
      global $db;
    $db = new \Workerman\MySQL\Connection('127.0.0.1', '3306', 'root', '123456', 'delicacy');
   }

 public static function onMessage($client_id, $message)
   {
   
    //查询数据库
    // $db->query("SELECT ID,Sex FROM `Persons` WHERE sex='M' AND ID = 1");
    // $db = new \Workerman\MySQL\Connection('127.0.0.1', '3306', 'root', '123456', 'delicacy');  
    $data = $db->query("select cno,cname from u_user");
    Gateway::sendToClient($client_id,json_encode($data));
    print_r($data);
 
   }