Session time out in PHP

1.
session_start();
2.

3.
// set timeout period in seconds
4.
$inactive = 600;
5.

6.
// check to see if $_SESSION['timeout'] is set
7.
if(isset($_SESSION['timeout']) ) {
8.
$session_life = time() - $_SESSION['timeout'];
9.
if($session_life > $inactive)
10.
{ session_destroy(); header("Location: logoutpage.php"); }
11.
}
12.
$_SESSION['timeout'] = time();