For thoses running Linux with apache2 you need to install php-mysql
apt-get install php-mysql
or if you are running ubuntu 16.04 or higher just running the following command will be enought, no need to edit your php.ini file
apt-get install php7.2-mysql
If you are running ubuntu 15.10 or below:
Edit your php.ini file, it’s located at /etc/php/[version]/apache2/php.ini
and search for pdo_mysql
you might found something like this
;extension=pdo_mysql.so
Change it to this
extension=pdo_mysql.so
Save the file and restart apache
service apache2 restart
Check that it’s available in your phpinfo()