[chamchoum]

The comments in the config file include the solution to your problem:

PHP Code:

sorry i'm new and i didn't understand what does it mean absolute path.

shall i change 
$config['sess_save_path'] = NULL;

Thanks
/*
...
| 'sess_save_path'
|
|    The location to save sessions to, driver dependent.
|
|    For the 'files' driver, it's a path to a writable directory.
|    WARNING: Only absolute paths are supported!
|
...
*/ 


Use an absolute path, not a relative path. In Linux, most absolute paths will start with / (in Windows, with a drive letter, like C:\). The easiest way to get an absolute path is to use something like APPPATH, BASEPATH, or FCPATH as the base of your path, or use PHP's realpath().