Cart Cannot Update Accordingly?

Dear Mr/Ms

Below attached the script.

When i choose the "Save Changes" button the total item and total price will always reset to "0", cannot add up. This problem also same when i change the quantity and press enter + press check out button.

Need your kind & expertize advice. Thank U so much.


show_cart.php

<?php
include ('book_sc_fns.php');
// The shopping cart needs sessions, so start one
session_start();

@ $new = $_GET['new'];

if($new)
{
//new item selected
if(!isset($_SESSION['cart']))
{
$_SESSION['cart'] = array();
$_SESSION['items'] = 0;
$_SESSION['total_price'] ='0.00';
}

if(isset($_SESSION['cart'][$new]))
$_SESSION['cart'][$new]++;
else
$_SESSION['cart'][$new] = 1;

$_SESSION['total_price'] = calculate_price($_SESSION['cart']);
$_SESSION['items'] = calculate_items($_SESSION['cart']);
}

if(isset($_POST['save']))
{
foreach ($_SESSION['cart'] as $isbn => $qty)
{
if($_POST[$isbn]=='0')
unset($_SESSION['cart'][$isbn]);
else
$_SESSION['cart'][$isbn] = $_POST[$isbn];
}
$_SESSION['total_price'] = calculate_price($_SESSION['cart']);
$_SESSION['items'] = calculate_items($_SESSION['cart']);
}

do_html_header('Your shopping cart');

if($_SESSION['cart']&&array_count_values($_SESSION['cart']))
display_cart($_SESSION['cart']);
else
{
echo '<p>There are no items in your cart</p>';
echo '<hr />';
}
$target = 'index.php';

// if we have just added an item to the cart, continue shopping in that category
if($new)
{
$details = get_book_details($new);
if($details['catid'])
$target = 'show_cat.php?catid='.$details['catid'];
}
display_button($target, 'continue-shopping', 'Continue Shopping');

// use this if SSL is set up
// $path = $_SERVER['PHP_SELF'];
// $server = $_SERVER['SERVER_NAME'];
// $path = str_replace('show_cart.php', '', $path);
// display_button('https://'.$server.$path.'checkout.php',
// 'go-to-checkout', 'Go To Checkout');

// if no SSL use below code
display_button('checkout.php', 'go-to-checkout', 'Go To Checkout');


do_html_footer();
?>