Add Mysqli to your older opencart versions BEFORE 1.5.6.1!
Starting from the opencart 1.5.6.1 use the native mysqli db driver.
This is a (FREE!) db "driver".
Please, do not expect (and pretend) an automatic installation or free support!
----------------INSTALLATION----------------
copy the system folder in your web folder
edit your config.php and /admin/config.php
change the
define('DB_DRIVER', 'mysql');
define('DB_DRIVER', 'dbmysqli');
It works with opencart version 1.5.5.1 AND before.