Set session variable if link is clicked, do not want to use GET

Hi! I am writing a simple shopping cart system which uses sessions and PHP. I would like a normal link that says "Add to cart" which the user can click and the item is added to the cart. However, i do not want to use GET for this as it could introduce unwanted features if the user bookmarks the page just after clicking "Add to cart". So i don't want an URL like: http://thisistheurl.com?buy=10 and therefore prefer if this could be done using POST. It would have been easy if i would have used a normal FORM and would have had a button called "Add to cart". But i would like it to be in url-style or using some <img ..> which i understand that the BUTTON item does not support (or am i wrong?). Any advice would be greatly appreciated. Best regards, Kent