how is $cart registered

[seanpaulcail]

Coming from a page "show_book.php"

we have the following button to add an item to the cart

display_button("show_cart.php?new=$isbn", "add-to-cart", "Add ".$book["title"]." To My Shopping Cart");

when this button is pressed we go to show_cart.php" with the isbn being passed as a parameter.

example of url: show_cart.php?new=0672317842

Show_Cart.php begins with the following code

if($new)
  {
    //new item selected
    if(!session_is_registered("cart"))
    {
      $cart = array();
      session_register("cart");
      $items = 0;
      session_register("items");
      $total_price = "0.00";      

      session_register("total_price");
    }
    if($cart[$new])
      $cart[$new]++;
    else 
      $cart[$new] = 1;
    $total_price = calculate_price($cart);
    $items = calculate_items($cart);


if($cart&&array_count_values($cart))
    display_cart($cart);
  else
  {
    echo "<p>There are no items in your cart";
    echo "<hr>";
  }

How is it that cart is registered when a book is added to the cart?
Please explain this code in laymen terms.

you can view the project @ http://www.caillouette.com/CHAPTER26a/show_book.php?isbn=0672317842

thanks