如何在有限的时间后过期购物车

I need Help on how to expire a shopping cart
Suppose we have a shopping cart in a e-commerce website, and a user adds a product to his shopping cart the problem is that from the inventory we should decrement the quantity of that product because it’s being held by a user.
The idea is we implement a timer in the shopping cart when that timer expires, the user doesn’t hold that product anymore.

My questions are :

• How to implement a timer that will work across all pages in the website
• How to trigger the event of updating the inventory once the timer is done.

Hope my explanation was clear. Thank you for your time.

One example using session, don't forget that the session will expire when the browser is closed. If you want more persistant data storage, use cookies or localStorage.

<?php

/* Constants */
define('EXPIRATION_TIME', 30); // minutes

/* Dummy variables */
$productAdded = true;

/* Start session */
session_start();

/* Check timer */
if (isset($_SESSION['timer']) && $_SESSION['timer'] < time()) {

    /* 
        30 min have gone by and the user has not added more products
        to the cart, lets empty the cart and reset the timer
    */

    unset($_SESSION['cart']);
    unset($_SESSION['timer']);
}

/* Add product */
if ($productAdded) {

    /* Increase timer */
    $_SESSION['timer'] = (time() + (EXPIRATION_TIME * 60));

    /* Add product to cart, and all other tasks */
    if (!isset($_SESSION['cart']))
        $_SESSION['cart'] = array();

    $_SESSION['cart'][] = array(
        'id' => 17,
        'name' => 'Fancy shampoo',
        'quantity' => 1337
    );
}

?>