shopping cart using php4 sessions

Anon

My object Cart is being reinstantiated at a bad time. I've searched like mad and read a lot, but evidently I'm still missing something about sessions. Somebody please take pity on me.

Here's a testing version. Sorry about the crazy formatting.

application.php:
<?
include("cart.php");

    session_start();
    session_register("SESSION");

    if (!isset($SESSION)) {
            echo "\$SESSION !isset, setting.<br>\n";
            $SESSION = array();
    } else {
            echo "\$SESSION isset<br>\n";
    }

    if (!isset($SESSION["cart"])) {
            echo "\$SESSION[\"cart\"] !isset, setting.<br>\n";
            $SESSION["cart"] = new Cart;
    } else {
            echo "\$SESSION[\"cart\"] isset<br>\n";
    }

index.php:
<?
include("application.php");
?>

cart.php:
<?
class Cart
{
var $items;

            function Cart()
            {
                    $this->init();
            }

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

            function set($id, $q)
            {
                    $this->items[$id] = $q;
            }

            function add($id)
            {
                    isset($this->items[$id]) ?
                            $this->items[$id] += 1 :
                            $this->items[$id] = 1;
            }

            function display()
            {
                    for ($i=0; $i<10; $i++) {
                            if (isset($this->items[$i])) {
                                    printf("%d=%d<br>\n", $i, $this->items[$i]);
                            }
                    }

                    echo "<br>\n";
            }
    }

shopping_cart.php:
<?
include("application.php");

    switch($a) {
            case "A":
                    echo "add.  id=$id<br>\n";
                    $SESSION["cart"]->add($id);
                    break;
    }

    $SESSION["cart"]->display();