persistant shopping card session destroy

Hi, I have built the "persistant shopping cart" in the tutorial and it works fine.
I want to however destroy the session when the user "checks out" but seem
to be getting totally lost with it. The session cookie is passed from page to page using:

function GetCartId()
{
// This function will generate an encrypted string and
// will set it as a cookie using set_cookie. This will
// also be used as the cookieId field in the cart table

if(isset($_COOKIE["cartId"]))
{
return $_COOKIE["cartId"];
}
else
{
// There is no cookie set. We will set the cookie
// and return the value of the users session ID

session_start();
setcookie("cartId", session_id());
return session_id();
}
}

which is stored in a file called db.php. The session is ended if they close the browser
but i need to destroy it incase they don't as my orders table relies on unique cookie
id's. Any help much appreciated

You should be able to do several things to help make this happen. First, check out session_unset() and session_destroy() and any other related functions at php.net. As these are sometimes flakey, you might also be able to set the cartId cookie again with a different value than the results of session_id() (for example, "NULL"). I'm not positive that one'll work, but it's worth a shot.

I have had a look at manual and google and sort of think i am on the right track with this:
On the top of a new page i have this:

<?php
session_start();
session_unset ();
session_destroy();
setcookie("cartId", "", time() - ((3600 * 24) * 2));
header ("location:stamps.php");
?>

however when i reload the cart the items are still there
meaning that its either my mistake or some caching problem. What i am really
unsure about is as i originally set a session_id can the session_unset recognise it if
you see what i mean.

Maybe pass session_id() to session_unset() and session_destroy()?

I override the default session handler and stick my sessions in a database. This allows me to manually delete sessions when I want to, which lets me get around any flakiness in session_unset() and session_destroy(). Maybe you'd consider doing something like that?