Destroying the session if the user is idle - codeigniter

Question

My goal is to destroy the logged-in used session and force him to log in again if he was idle for 20 minutes. Here is my way of doing it:
In each controller, I do this check:
if(reach_idle_limit()) {
redirect('logout');
}
And reach_idle_limit() is a helper method in one of my helper classes:
function reach_idle_limit() {
$idle_period = 1200; //20 mins
$CI =& get_instance();
$last_activity = $CI->session->userdata('last_activity');
$now_time = time();
//If $last_activity is not set, don't force a logout
if($last_activity == False || $last_activity == 0){
return false;
}
//If idle period exceeded: destroy the session and return true
else if($now_time - $last_activity > $idle_period){
$CI->session->sess_destroy();
return true;
}
//else, update session's last_activity to current time, return false
else{
$CI->session->set_userdata('last_activity', $now_time);
return false;
}
}
This works fine when I give $idle_period a small value, like 60 sec. But when I give it the value I seek, 20 min, it doesn't work!
FYI:
I'm using ag_auth library with Codeigniter (for the authentication part).
My config's variable sess_expiration is set to 0.

Answer 1

In this case, why not use session.gc_maxlifetime?
session.gc_maxlifetime specifies the number of seconds after which data will be seen as 'garbage' and potentially cleaned up. Garbage collection may occur during session start (depending on session.gc_probability and session.gc_divisor).
just use:
ini_set('session.gc_maxlifetime',20);