automatic logout after x minutes activity - php

Question

I have php scripts that confirm the password and username of a customer matches those that are stored in a database (username and password are stored in a users database). Once they login they will be redirected to their own page, but after a period of minutes inactivity I want them to be logged out, or redirected to the login.php. Can anyone help with a tried and tested method? Thank you in advance.
<?php
session_start();
$username = mysql_real_escape_string($_POST['username']);
$password = mysql_real_escape_string($_POST['password']);
mysql_connect("localhost", "root","root") or die(mysql_error()); //Connect to server
mysql_select_db("first_db") or die("Cannot connect to database"); //Connect to database
$query = mysql_query("SELECT * from users WHERE username='$username'"); //Query the users table if there are matching rows equal to $username
$exists = mysql_num_rows($query); //Checks if username exists
$table_users = "";
$table_password = "";
if($exists > 0) //IF there are no returning rows or no existing username
{
while($row = mysql_fetch_assoc($query)) //display all rows from query
{
$table_users = $row['username']; // the first username row is passed on to $table_users, and so on until the query is finished
$table_password = $row['password']; // the first password row is passed on to $table_users, and so on until the query is finished
$table_id = $row['id'];
$page_id = $row['page'];
}
if(($username == $table_users) && ($password == $table_password)) // checks if there are any matching fields
{
if($password == $table_password)
{
$_SESSION['user'] = $username; //set the username in a session. This serves as a global variable
//echo $table_id;
//echo $page_id;
redirect ($page_id); //take the user to the page specified in the users table
}
else
{
echo "Login Failed";
}
}
else
{
Print '<script>alert("1. Incorrect Password!");</script>'; //Prompts the user
Print '<script>window.location.assign("login.php");</script>'; // redirects to login.php
}
}
else
{
Print '<script>alert("Incorrect Username!");</script>'; //Prompts the user
Print '<script>window.location.assign("login.php");</script>'; // redirects to login.php
}
function redirect($page_id)
{
/* Redirect browser */
header('Location: home.php');
/* Make sure that code below does not get executed when we redirect. */
exit;
}
?>

Answer 1

Add this to your pages :
setcookie(session_name(), $_COOKIE[session_name()], time() + 15 * 60); // 15 is the number of minutes of inactivity before the session gets destroyed
This tells the session that it has still X minutes left before dying.

Answer 2

In your page heading, add a meta that will redirect the user after a giving time
<meta http-equiv="refresh" content="900;url=logout.php" />