session storage

i am writing a shopping site for training (just out of interest). When a user clicks "add to basket" i want to check whether an item with that id is already in my session ($ALCOHOLE["items"]), if so, redirect them the the view_cart page, if not then append it to the $ALCOHOLE["items"] array as $key => $value pairs. I have played around with all sorts of loops and sorts all day but still cannot get it sorted. Can anyone suggest where I am going wrong or has anyone got a better method they would care to suggest? cheers.