Help with shopping cart

[mikawhat]

This is the code of the shopping cart i'm using:

	$cart = $_SESSION['cart'];
	$action = $_GET['action'];
	switch ($action) {
		case 'add':
			if ($cart) {
				$cart .= ','.$_GET['id'];
			} else {
				$cart = $_GET['id'];
			}
			break;
		case 'delete':
			if ($cart) {
				$items = explode(',',$cart);
				$newcart = '';
				foreach ($items as $item) {
					if ($_GET['id'] != $item) {
						if ($newcart != '') {
							$newcart .= ','.$item;
						} else {
							$newcart = $item;
						}
					}
				}
				$cart = $newcart;
			}
			break;
		case 'update':
		if ($cart) {
			$newcart = '';
			foreach ($_POST as $key=>$value) {
				if (stristr($key,'qty')) {
					$id = str_replace('qty','',$key);
					$items = ($newcart != '') ? explode(',',$newcart) : explode(',',$cart);
					$newcart = '';
					foreach ($items as $item) {
						if ($id != $item) {
							if ($newcart != '') {
								$newcart .= ','.$item;
							} else {
								$newcart = $item;
							}
						}
					}
					for ($i=1;$i<=$value;$i++) {
						if ($newcart != '') {
							$newcart .= ','.$id;
						} else {
							$newcart = $id;
						}
					}
				}
			}
		}
		$cart = $newcart;
		break;
	}
	$_SESSION['cart'] = $cart;

This cart is pretty good when having a simple e-shop with 1 price per product. However, now i'm working on a site that products have different prices according to size, and different colors. What i'm thinking is to somehow when adding a product to add it multiple times for each size/price and color, so that when added into the basket the id will be different. I don't really know if this is a solution that will work, but it's the only way i can think of.... Can anybody suggest another solution for this issue?

thanks!