使用CodeIgniter 2.1.3加载数据库库的问题

I am using CodeIgniter, and i want to add a functionality in someone else's project by accessing the database.
When i automatically load the database library in the autoload.php file this way,

$autoload['libraries'] = array('doctrine','session','form_validation','database');

have an error starting like this

Fatal error: Uncaught Error: Call to undefined function mysql_pconnect()

How can I fix that ?

Change dbdriver to MySQLi

Path application/config/database.php

$db['default']['hostname'] = 'localhost';
$db['default']['username'] = 'root';
$db['default']['password'] = '';
$db['default']['database'] = '';
$db['default']['dbdriver'] = 'mysqli'; # Change this