PHP and MySQL script problem. Please help, urgent!

Hi, I'm building a shopping cart using PHP and MySQL. I've done most of the stuff needed but I am having a problem when it comes to adding items to the cart. If I click add to cart, the program says it has done so. But when I go to the cart, it says that it is empty. I have a feeling I've done something wrong with the sessions or with the MySQL query. Could anyone please help me? Here's the code for the addcart.php file:


<?php

$page_title = 'Add to Cart';

if (isset ($_GET['pid'])) {

$pid = (int) $_GET['pid'];

if (isset($_SESSION['cart'][$pid])) {

$_SESSION['cart'][$pid]['quantity']++; // Add another.

// Display a message.
echo '<p>Another copy of the print has been added to your shopping cart.</p>';

} else { // New product to the cart, get the price information.

require_once ('sqlconnect.php'); // Connect to the database.

$query = "SELECT price FROM product WHERE product.product_id = $pid";
$result = mysqli_query ($dbc, $query);

if (mysqli_num_rows($result) == 1) {

list($price) = mysqli_fetch_array ($result, MYSQLI_NUM);

$_SESSION['cart'][$pid] = array ('quantity' => 1, 'price' => $price);

echo '<p>The product has been added to your shopping cart. If you want to purchase travel for this event, please <a href="browse_travel.php"> Go Here </a> Otherwise, go straight to the <a href="view_cart.php">shopping cart </a></p>';

} else {
echo '<div align="center">This page has been accessed in error!</div>';
}

mysqli_close($dbc);

} // End of isset($_SESSION['cart'][$pid] conditional.

} else { // No print ID.
echo '<div align="center">This page has been accessed in error!</div>';
}


?>

And here's the code for the shopping cart:


<?php

$page_title = 'View Your Shopping Cart';

if (isset($_POST['submitted'])) {

// Change any quantities.
foreach ($_POST['qty'] as $k => $v) {

// Must be integers!
$pid = (int) $k;
$qty = (int) $v;

if ( $qty == 0 ) { // Delete.
unset ($_SESSION['cart'][$pid]);
} elseif ( $qty > 0 ) { // Change quantity.
$_SESSION['cart'][$pid]['quantity'] = $qty;
}

} // End of FOREACH.
} // End of SUBMITTED IF.

// Check if the shopping cart is empty.
$empty = TRUE;
if (isset ($_SESSION['cart'])) {
foreach ($_SESSION['cart'] as $key => $value) {
if (isset($value)) {
$empty = FALSE;
break; // Leave the loop.
}
} // End of FOREACH.
} // End of ISSET IF.

// Display the cart if it's not empty.
if (!$empty) {

require_once ('sqlconnect.php');

// Retrieve all of the information for the prints in the cart.
$query = "SELECT * FROM category, product WHERE category.cat_id = product.cat_id AND product.product_id = $pid IN (";
foreach ($_SESSION['cart'] as $pid => $value) {
$query .= $pid . ',';
}
$query = substr ($query, 0, -1) . ') ORDER BY product.product_name ASC';
$result = mysqli_query ($dbc, $query);

// Create a table and a form.
echo '<table border="0" width="90%" cellspacing="3" cellpadding="3" align="center">
<tr>
<td align="left" width="30%"><b>Product Type</b></td>
<td align="left" width="30%"><b>Product Name</b></td>
<td align="right" width="10%"><b>Price</b></td>
<td align="center" width="10%"><b>Qty</b></td>
<td align="right" width="10%"><b>Total Price</b></td>
</tr>
<form action="view_cart.php" method="post">
';

// Print each item.
$total = 0; // Total cost of the order.
while ($row = mysqli_fetch_array ($result, MYSQLI_ASSOC)) {

// Calculate the total and sub-totals.
$subtotal = $_SESSION['cart'][$row['product_id']]['quantity'] * $_SESSION['cart'][$row['product_id']]['price'];
$total += $subtotal;

// Print the row.
echo " <tr>
<td align="left">{$row['cat_name']}</td>
<td align="left">{$row['product_name']}</td>
<td align="right">${$_SESSION['cart'][$row['product_id']]['price']}</td>
<td align="center"><input type="text" size="3" name="qty[{$row['product_id']}]" value="{$_SESSION['cart'][$row['product_id']]['quantity']}" /></td>
<td align="right">$" . number_format ($subtotal, 2) . "</td>
</tr>n";
} // End of the WHILE loop.

mysqli_close($dbc); // Close the database connection.

// Print the footer, close the table, and the form.
echo ' <tr>
<td colspan="4" align="right"><b>Total:<b></td>
<td align="right">$' . number_format ($total, 2) . '</td>
</tr>
</table><div align="center"><input type="submit" name="submit" value="Update My Cart" />
<input type="hidden" name="submitted" value="TRUE" />
</form><br /><br /><a href="checkout.php"><font size="+2">Checkout</font></a></div>';

} else {
echo '<p>Your cart is currently empty.</p>';
}

?>

Any help would be much appreciated, thanks!