PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

Implementing the Shopping Cart
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Listing 28.11
Continued
$result = $conn->query($query);
if ($result) {
$item = $result->fetch_object();
$item_price = $item->price;
$price +=$item_price*$qty;
}
}
}
return $price;
}
As you can see, the calculate_price() function works by looking up the price of each
item in the cart in the database.This process is somewhat slow, so to avoid doing this
more often than you need to, you store the price (and the total number of items, as well)
as session variables and recalculate only when the cart changes.
Listing 28.12 calculate_items() Function from book_fns.php—Function That
Calculates and Returns the Total Number of Items in the Shopping Cart
function calculate_items($cart) {
// sum total items in shopping cart
$items = 0;
if(is_array($cart)) {
foreach($cart as $isbn => $qty) {
$items += $qty;
}
}
return $items;
}
The calculate_items() function is simpler; it just goes through the cart and adds the
quantities of each item to get the total number of items using the array_sum() function.
If there’s not yet an array (if the cart is empty), it just returns 0 (zero).
Saving the Updated Cart
If the user comes to the show_cart.php script by clicking the Save Changes button, the
process is a little different. In this case, the user has arrived via a form submission. If you
look closely at the code, you will see that the Save Changes button is the submit button
for a form.This form contains the hidden variable save. If this variable is set, you know
that you have come to this script from the Save Changes button.This means that the
user has presumably edited the quantity values in the cart, and you need to update them.