PHP - make session expire after X minutes

Question

i am using the following technique...

From the login.php the form posts to the page check.php where i do this

Answer 1

Store a timestamp in the session:

 0)
{
    $array = mysql_fetch_assoc($result);    

    session_start();
    $_SESSION['user_id'] = $uzer;
    $_SESSION['login_time'] = time();
    header("Location:loggedin.php");            
}
else
{
    header("Location:login.php");
}
?>

Check if the timestamp is within the allowed time window (600 seconds is 10 minutes):

 600)
{
    header("Location:login.php");
}
else
{
    // uncomment the next line to refresh the session, so it will expire after ten minutes of inactivity, and not 10 minutes after login
    //$_SESSION['login_time'] = time();
    echo ( "this session is ". $_SESSION['user_id'] );
    //show rest of the page and all
}
?>