PHP: How can I perform actions related to a particular session on my db once a user has closed the browser?

I am doing a food shopping website. I give a random number to any visitor on the site generated by rand() and when they add to cart, i give the cartNo that particular visitor No. However, if a visitor has added to cart and not proceeded to checkout, the items will remain in the cart and the next time they open the browser, they wont ever get that particular cartNo to which their items were added. I want to empty the cart in case that scenario happens. How do I do that?