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.
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 INSTALL PHP70W-MYSQL
Once installed, restart your web server, for Apache it is:
systemctl restart httpd
service httpd restart
After restarting, your WordPress site should work properly.