thinkphp设置DB_CHARSET为utf8mb4

SQLSTATE[HY000] [2019] Can't initialize character set utf8mb4 (path: /usr/share/mysql/charsets/)
错误位置

FILE: /data1/www/htdocs/733/ecampus/3/ThinkPHP/Library/Think/Db/Driver.class.php 　LINE: 109

如果不用tp，使用原生的sql ‘set names utf8mb4’ 执行没问题。
到底哪里出错了 求解决！谢谢!