An uncaught Exception was encountered

My employer is closing down an existing webserver where we're running a few websites based on CodeIgniter 3.0.6. I recently backed up one of the sites and migrated it to a new web host, but am having difficulty getting the site running. I changed the server settings to use the /public directory as the home for the site, but it was not finding the application and system directories, so the hosting support said to change those variables to the full path. We did that, and now I'm getting an error message. I have no idea how to address this.

I found the /application/config/production directory and updated the config.php and database.php to match the new database's settings.

An uncaught Exception was encountered
Type: Error

Message: Call to undefined function mysql_connect()

Filename: /home/impact_nswtl1/nswtl.impactpreview.com/system/database/drivers/mysql/mysql_driver.php

Line Number: 136

Backtrace:

File: /home/impact_nswtl1/nswtl.impactpreview.com/application/controllers/Home.php
Line: 6
Function: __construct
File: /home/impact_nswtl1/nswtl.impactpreview.com/public/index.php
Line: 321
Function: require_once

***
here's line 136 in  /home/impact_nswtl1/nswtl.impactpreview.com/system/database/drivers/mysql/mysql_driver.php

: mysql_connect($this->hostname, $this->username, $this->password, TRUE, $client_flags);