nginx 504 Gateway Time-out

1.连接数据库失败,php warning,导致的
[28-Dec-2020 16:20:34 PRC] PHP Warning: mysqli::real_connect(): (HY000/2003): Can't connect to MySQL server on 'cq01-dr-ems08.cq01.baidu.com' (4) in /home/work/odp/php/phplib/bd/DB.php on line 246