How to fix CDbConnection failed to open the DB connection: could not find driver error on php.5.6 in ubuntu 18?

Question

linux

I'm trying to get yii1. ubuntu has php 7.2 installed. Apache webserver
installed 5.6 and switched:

sudo a2dismod php7.2 ; sudo a2enmod php5.6 ; sudo service apache2 restart
sudo update-alternatives --set php /usr/bin/php5.6

phpinfo outputs

PDO drivers mysql

But the page throws an error:

CDbConnection failed to open the DB connection: could not find driver