creating an automatic logout page in php - php

Question

i am trying to write a automatic logout script which seems to work but not to my expectations,i do not know what exactly im doing wrong,i want to put the timeout.php on every page so that when the user is idle it logs out automatically and redirects it to login page but when i put the timeout.php on for my add user page where admin adds users,its overriding the link for the add user page and putting a login page which is also not coming out nicely(i.e the form is getting out of its position)
this is the timeout.php code
<?php
$_SESSION = 0;
if($_SESSION['session_count'] == 0) {
$_SESSION['session_count'] = 1;
$_SESSION['session_start_time']=time();
} else {
$_SESSION['session_count'] = $_SESSION['session_count'] + 1;
}
$session_timeout = 10; // (in sec)
$session_duration = time() - $_SESSION['session_start_time'];
if ($session_duration > $session_timeout) {
session_unset();
session_destroy();
session_start();
session_regenerate_id(true);
$_SESSION["expired"] = "yes";
header("Location: login.php"); // Redirect to Login Page
} else {
$_SESSION['session_start_time']=time();
}
?>
i want it to automatically logout and redirect itself to the login page with a message yoour session has expired and i want it to put it on every page without it disturbing the forms or overlapping the page