help with script

[turbo99]

I have this function from a shopping cart class I am trying to modify. I passing the product_id (hidden input) and an array using a form. I would like to modify the code to add the array value. What do I need to do to make it work?

here is the function:

function add(&$productid, $qty) {
/ add an item to the shopping cart and update the total price /

	if (isset($productid)) {
		setdefault($this->items[$productid], 0);
		$this->items[$productid] += $qty;
	}
}

Also what is the ampersand in the arguement doing?

Here is the part of the cart class if you needed to see this info as well.

class Cart {
var $items; / array of items /
var $total; / total of the cart /

function Cart() {
/* object constructor */

	$this->init();
}

function init() {
/* this function is called to initialize (and reset) a shopping cart */

	$this->items = array();
	$this->total = 0;
}

function add(&$productid, $qty) {
/* add an item to the shopping cart and update the total price */

	if (isset($productid)) {
		setdefault($this->items[$productid], 0);
		$this->items[$productid] += $qty;
	}
}

function set(&$productid, $qty) {
/* set the quantity of a product in the cart to a specified value */

	if (isset($productid)) {
		$this->items[$productid] = (int) $qty;
	}
}

Thx in advance,

Alex