pdo_mysql removed on updating to 16.04

At this stage I would simply recommend "turning it off and on again" by doing:

sudo apt-get purge php-common php5-common

followed by

sudo rm -r /etc/php

And then reinstalling PHP 7.0 and mysql modules:

sudo apt-get install libapache2-mod-php php-mysql

You may also try just purging and reinstalling mysql modules, first removing the package:

sudo apt-get purge php7.0-mysql

And the installing them again:

sudo apt-get install php-mysql

In both cases it's important to use apt-get purge instead of apt-get remove to delete any existing configuration files and package states.

The least destructive way would be trying to re-enable mysql modules using phpenmod command:

sudo phpenmod mysqlnd pdo_mysql

It's hard to give you correct advice if we don't know what mayhem you created on your system.