Several kinds of drivers can be used to connect to MySQL database, namely - MySQL, MySQLi, PDO.
Changing the database driver must be done directly in config file site by changing the line:
$database_type = 'XXXXX';
Instead XXXXX
you need to specify a suitable driver.
MySQL
not supported in PHP 5.5 and higher, and has been completely removed since version 7.0. For more details, this information is provided on official website.