rewriting to a cookie

[Anon]

Hi All I want to make a cookie that hold a value like a shopping cart would do I click on a button and add the vaule to the cookie but want I can't seem to do is hold the first value I stored and add another

setcookie("items_tray[$total_items]", $cat_id);

setcookie("quantity[$total_items]", "1");
How can I know if the item are all still present in these cookies

So I can hold 20 items in this case 20 cat_id numbers

Thanks for any help