ON CENTOS
If you are running your WordPress site on a CentOS VPS without any kind of control panel. Your first step would be to check the PHP version that you are running.
PHP -V
Will show you the PHP Version you are currently running. In our case, this is PHP 7.0. We need to install the PHP-MySQL extension for PHP 7.0. We can do this using the following command:
YUM UPDATE
YUM INSTALL PHP70W-MYSQL
Once installed, restart your web server, for Apache it is:
systemctl restart httpd
Or
service httpd restart
After restarting, your WordPress site should work properly.