Adding an additional session variable

[micmac]

Hi all,
If i have the following script:

include ('functions.php');


session_start();

if (!empty($_POST['movie_id'])) {


			$movie_id = $_POST['movie_id'];

	// Starting a new cart session
	if (!session_is_registered("cart")) {
		$_SESSION['cart'] = array();
		$_SESSION['items'] = 0;
		$_SESSION['total_price'] = 0.00;		
	}

	// Existing DVD in the cart + 1
	if (!isset($_SESSION['cart'][$movie_id])) {
		$_SESSION['cart'][$movie_id]++;	
	}
	else {
		$_SESSION['cart'][$movie_id] = 1;
	}	

	$_SESSION['total_price'] = calculate_price($_SESSION['cart']);
	$_SESSION['items'] = calculate_items ($_SESSION['cart']);

    if ($_POST['ord_ship_cost']){
	$_SESSION['ord_ship_cost'] = $_POST['ord_ship_cost'];
	}

	header ("Location: ./");

}

how can i add the variable $_SESSION['ship_cost'] to the cart session array? The vlaue is coming from a form.

Cheers,
micmac