Codeigniter Code Injected in Index.php

I have a

Code:

codeigniter

project and I see the

Code:

index.php

page keep getting hacked :-
Some code is added automatically every time on the top of the index.php page and my site is redirect on third party url whenever i click anywhere
The injected code as follows :

Code:

error_reporting(0);
ini_set("display_errors",0);
include_once(sys_get_temp_dir()."/SESS_48cd7517d21176f980daa5502d9efb31");

How can I prevent this?