Severity: error --> Exception: Call to undefined function mysqli_init() code example

Example: Call to undefined function mysqli_init()

it is not a bug in your application, it is just a missing driver, so, you have
couple of options...

Go to your php init and uncomment the following:

extension=php_mysqli.dll
If not, try installing it at your server, it varies depending on your 
distribution.

Try installing php5-mysqlnd

If you cannot do it by hosting restrictions then just move to mysql driver 
(wont need to change other configurations or queries in CodeIgniter or anything
 else...)

like this (at your config file)

$db['default']['dbdriver'] = 'mysql'; (you might have mysqli now)