1, update the database similar to below:
INSERT INTO collection (user_id, item_id)
VALUES ($_SESSION[id], $_GET[id])
... where $_SESSION[id] is the ID of the logged on user, 1 indicates it is in their collection and $_GET[id] is the item that is being marked as part of the user's collection.
2. If the tickbox is already ticked and they wish to remove it from their collection it would instead do:
DELETE FROM collection WHERE user_id = $_SESSION[id] AND item_id = $_GET[id] LIMIT 1
3. The tickbox image should automatically update to ticked or unticked status according to whether it is in the user's collection after the onclick trigger.
IMPORTANT: This should be done without reloading the page (presumably by AJAX.)
Examples of similar functionality can be found at Amazon (you can add items to your wishlist in this way) or at www.icheckmovies.com.
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The rest of this page is already written - this is simply adding the above functionality. I will create a suitable MySQL table in advance.