ERROR AL PASAR http://localhost/ch_08/public_files/cart.php?update

[taldigi]

Quiero hacer una aplicacion de carrito de compra , aprovechando un codigo fuente ya preconstruido.
Pero me salen errores , uno de ellos no puedo solucionar.
¿Si alguien puede ayudarme?

me sale este error al intentar hacer update :
http://localhost/ch_08/public_files/cart.php?update

El sitio web ha detectado un error al recuperar la http://localhost/ch_08/public_files/cart.php?update http://localhost/ch_08/public_files/cart.php?update. Es posible que esta esté inactiva debido a tareas de mantenimiento o que se haya configurado de forma incorrecta.

Error de HTTP 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.

EL SCRIPT ES :


<?
include '../lib/common.php';
include '../lib/db.php';
include '../lib/ShoppingCart.php';

// create or resume session and retrieve shopping cart
session_start();
if (isset($_SESSION['cart']))
{
$cart = unserialize($_SESSION['cart']);
}
else
{
$cart = new ShoppingCart();
}

// empty the shopping cart and redirect user to list of categories
if (isset($_GET['empty']))
{
$cart->removeAll();
$_SESSION['cart'] = serialize($cart);
header('Location: shop.php');
end();
}

// item parameter indicates an attempt to add or remove items
if (isset($_GET['item']))
{
// verify item is valid
$query = sprintf('SELECT ITEM_ID FROM %sSHOP_INVENTORY WHERE ' .
'ITEM_ID = %d',
DB_TBL_PREFIX,
$_GET['item']);
$result = mysql_query($query, $GLOBALS['DB']);

if (mysql_num_rows($result))
{
$row = mysql_fetch_assoc($result);
$item = $row['ITEM_ID'];

// add item to cart
if (isset($_GET['add']))
{
$cart->addItem($item);
}

// remove item from cart
else if (isset($_GET['remove']))
{
$cart->removeItem($item);
}
}
mysql_free_result($result);

// save cart to session and redirect to the previously viewed page
$_SESSION['cart'] = serialize($cart);
header('Location: ' . htmlspecialchars($_SERVER['HTTP_REFERER']));
exit();
}

// view shopping cart's contents
else
{
// update item quantities in shopping cart
if (isset($_GET['update']))
{
foreach ($_POST['qty'] as $item => $qty)
{
$cart->addItem($item, $qty);
}
}

ob_start();

echo '<h1>Your Cart</h1>';
echo '<p><a href="shop.php">Back to all categories</a>';

// verify category parameter and construct suitable back link if passed
if (isset($_GET['category']))
{
$query = sprintf('SELECT CATEGORY_ID, CATEGORY_NAME FROM ' .
'%sSHOP_CATEGORY WHERE CATEGORY_ID = %d',
DB_TBL_PREFIX,
$_GET['category']);
$result = mysql_query($query, $GLOBALS['DB']);

if (mysql_num_rows($result))
{
$row = mysql_fetch_assoc($result);
echo ' / <a href="shop.php?category=' . $row['CATEGORY_ID'] .
'">Back to ' . $row['CATEGORY_NAME'] . '</a>';
}
mysql_free_result($result);
}
echo '</p>';

if ($cart->isEmpty)
{
echo '<p><b>Your cart is empty.</b></p>';
}
else
{
// display empty cart link
echo '<p><a href="cart.php?empty">';
echo '<img src="img/cartempty.gif" alt="Empty Cart"/></a></p>';

// encapsulate list in form so quantities may be changed
echo 'dd <form method="post" action="cart.php?update"';
// if a category was passed and was validated successfully earlier
// then append it to the action url so the back link remains available
if (isset($row['CATEGORY_ID']))
{
echo '&category=' . $row['CATEGORY_ID'];
}
echo '">';

// list each item in the cart, keeping track of total price
$total = 0;
echo '<table>';
echo '<tr><th>Item</th><th>Qty</th><th>Price</th><th>Total</th></tr>';
foreach ($cart->contents as $id => $qty)
{
$query = sprintf('SELECT ITEM_NAME, PRICE FROM %sSHOP_INVENTORY ' .
'WHERE ITEM_ID = %d',
DB_TBL_PREFIX,
$id);
$result = mysql_query($query, $GLOBALS['DB']);

$row = mysql_fetch_assoc($result);
echo '<tr>';
echo '<td><a href="shop.php?item=' . $id . '">' . $row['ITEM_NAME'] .
'</a></td>';
echo '<td><select name="qty[' . $id . ']">';
for ($i=0; $i < 11; $i++)
{
echo '<option ';
if ($i == $qty)
{
echo 'selected="selected" ';
}
echo 'value="' . $i . '">' . $i . '</option>';

}
echo '</td>';
echo '<td>$' . number_format($row['PRICE'], 2) . '</td>';
echo '<td>$' . number_format($row['PRICE'] * $qty, 2) . '</td>';
echo '</tr>';

$total += $row['PRICE'] * $qty;
mysql_free_result($result);
}
echo '</table>';
echo '<input type="submit" value="Update"/>';

echo '<p>Total Items: ' . $cart->totalItems . '<br/>';
echo 'Total Quantity: ' . $cart->totalQty . '</p>';
echo '<p><b>Total Price: $' . number_format($total, 2) . '</b></p>';

// display link to checkout
echo '<p><a href="checkout.php">';
echo '<img src="img/checkout.gif" alt="Proceed to Checkout"/></a></p>';
}

// save cart to session and display the page
$_SESSION['cart'] = serialize($cart);

$GLOBALS['TEMPLATE']['content'] = ob_get_clean();
include '../templates/template-page.php';
}
?>