//define the current time
$current = time();
//then we decided how long the limit, 60s X 30
$limit = $current – 60 * 30;

//this is for check how long last user activity
if (isset ($_SESSION['last_activity']) &&
$_SESSION['last_activity'] < $limit) {

//now the rule of limit time will work and redirect if time limit expired
$_SESSION = array();
header('Location: http://yoursite/logout.php‘);
exit;
} else {

//But if time not yet expired, then user will still logged in
$_SESSION['last_activity'] = $current;