Codeigniter deprecate Message After Moving From 2.1 to 3.0

I recently got this script of the web for ajax dropdown populate fields (v 2.1)

I works fine if run with same version but if I try to upgrade to 3.0, I get this error:

A PHP Error was encountered
Severity: 8192
Message: mysql_pconnect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead
Filename: mysql/mysql_driver.php
Line Number: 135

But the script still works.

I don't understand this error as I can't find mysql_driver.php or mysql_pconnect either. Please help.