I've developed a shopping cart with PHP and MySql which works fine. Once an item is added to the cart it is withdrawn from the database instead of leaving the withdrawing till the checkout stage. This avoids shoppers all choosing an item that may be the last in stock. The only problem is if the shopper that has the item in their cart decides not to buy and leaves the site without removing the item from the cart or the session times out, the item hasn't been purchased but also hasn't been put back in the database. What can i do to get round this?

Run a script which checks the cart. If the cart is abondoned by a visitor and if there are any items in the cart, you can add them back to database. You can write a script and run it when the browser is closed or session is timed out. Also you can run the script automatically using cron jobs.

Postpone inventory deduction until checkout - if a user attempts to checkout with an item which just sold out, offer a contact form to ensure the customer has the option to complete the sale through a representative (this is generally the best way to prevent a sale lost to understocking, if your representative is good over the phone or by e-mail).