原因：

PHP中 mysql.default_socket、mysqli.default_socket、pdo_mysql.default_socket没有检测到MySQL socket 路径

解决方案:

1、查看phpinfo信息中mysql.default_socket、mysqli.default_socket、pdo_mysql.default_socket 中有没有MySQL socket 路径

vim  info.php
<?php
  phpinfo();
?>

2、查看MYsql 数据库的MySQL socket 路径位置

mysql -u root -p密码

mysql> status;
 --------------
 mysql Ver 14.14 Distrib 5.7.20, for Linux (x86_64) using EditLine wrapper

Connection id: 31
 Current database:
 Current user: root@localhost
 SSL: Not in use
 Current pager: stdout
 Using outfile: ''
 Using delimiter: ;
 Server version: 5.7.20 Source distribution
 Protocol version: 10
 Connection: Localhost via UNIX socket
 Server characterset: utf8
 Db characterset: utf8
 Client characterset: utf8
 Conn. characterset: utf8
 UNIX socket: /usr/local/mysql/mysql.sock    #socket路径
 Uptime: 4 hours 54 min 47 sec

Threads: 1 Questions: 242 Slow queries: 0 Opens: 137 Flush tables: 1 Open tables: 55 Queries per second avg: 0.013
 --------------

3、修改php.ini配置(查找修改)

vim php.ini
mysql.default_socket =  /usr/local/mysql/mysql.sock
mysqli.default_socket =  /usr/local/mysql/mysql.sock
pdo_mysql.default_socket =  /usr/local/mysql/mysql.sock

4、重启服务

systemctl restart nginx.service
systemctl restart php-fpm.service

5、访问info.php查看结果