Your PHP installation appears to be missing the MySQL extension which is required by WordPress

Quick Fix: Update WordPress: WordPress dashboard ==> login ==> go to “Updates” ==> “Update Now” button

This is likely caused by having a newer version of PHP which isn’t compatible with an outdated MySQL extension. This extension was removed as of PHP 7.0. You will need to upgrade WordPress along with any plugins.

This could be caused by either you or your hosting provider upgrading PHP on your host.

You can check which version of PHP you have by typing the following on the command line ( assuming you have access to this ):

php -v

You can also check your PHP version by creating a PHP file like this in your web directory and accessing it with a web browser:

info.php
<?php phpinfo(); ?>

The Fix

The fix for this is probably going to be to update WordPress.

WARNING - Always back anything up before making any system changes.

The easiest way to do this is probably going to be to use your WordPress dashboard. Just login and go to “Updates”. There should be an “Update Now” button that should make things fairly easy.

If you are using managed hosting and you installed wordpress through your hosting provider then you will likely have an option to upgrade wordpress through whatever dashboard or panel that they provide. I would personally prefer the actual WordPress dashboard but this is also a valid option.

If you have access to a terminal on your server and the tools are installed you may be able to just run the following WordPress command to update word press. I’ve never done this myself but it seems like it would probably be easy enough.

wp core update

You should be able to update plugins from within WordPress.

If you do happen to be running an old version of PHP and you either don’t want to update it or can’t update it for whatever reason you might be able to fix this problem by installing the PHP MySQL package. You may be able to do this using the following commands but you really should be using a newer version of PHP if at all possible.

Ubuntu:

sudo apt-get update
sudo apt-get install php5-mysqlnd

Red Hat/CentOS:

sudo yum update
sudo yum install php-mysql

Here is a video in which someone talks about fixing this error: