Storing multiple values in a $_SESSION variable with PHP

I'm creating a site which has a shopping cart. I do not need any special functionality so I'm creating the cart on my own rather than integrating any ready one. My products do not have a predefined price in the database. The price is being generated dynamically based on the values entered by a user on the product page. So, the user chooses some specifications, enters the quantity and I get the following values:

Item ID
Quantity
Total price

I need to store those values in the $_SESSION variable and then loop over it when needed to get the results and print them in the shopping cart. The problem is that there are a lot of products and I need to store all those values (Quantity, Total Price) distinctively for the chosen product. That said, how do I store Item ID, Quantity and Total price in the $_SESSION variable and associate those values with each other?

Thanks for helping.

EDIT: My code implementing Michael's suggestions:

$itemid = $db->escape($_POST['productid']);
    $itemquantity = $db->escape($_POST['itemquantity']);
    $totalprice = $db->escape($_POST['totalprice']);

    $_SESSION['items'] = array();

    $_SESSION['items'][$itemid] = array('Quantity' => $itemquantity, 'Total' => $totalprice);

    var_dump($_SESSION);