codeigniter sess_expire_on_close not working

[eluser]mouna[/eluser]
I'm trying to implement a login form with a "remember me" checkbox

the problem is when I set "sess_expire_on_close" in the config file to true

Code:

$config['sess_expire_on_close'] = TRUE;

when I close the browser, it dosen't work, I always have access like if I'm logged

I'm using codeigniter 2.1.2

Any help will be appreciated.