在自己定义的类里调用workerman自带Mysql 类的问题

class Db {
    public static function getDbInstance()
    {
         if (!self::$db) {
             self::$db = new Workerman\MySQL\Connection('127.0.0.1', '3306', 'root', '', 'mysqlName');
         }
         return self::$db;
    }
}

Db::getDbInstance()->select(..)....