Session Destroy for persistant shopping cart

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 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

edit - realise i have posted here instead of php forum, sorry