PHP Notice: Undefined offset: 36 - php

Question

I have a simple php checkout but when adding the product to basket i get the following error:
PHP Notice: Undefined offset: 36 in C:\inetpub\wwwroot\shopping\cart.php on line 76
If i then refresh the page or go back to add another product it displays a product fine, it seems to be when adding to first product to the cart we get this error. any help would be much appreciated as all the answers i have tried have not worked.
<?php
// connect to the database
include("databasedrop.php");
?>
<div class="post">
<h2>Shopping Basket<span class="title-bottom"> </span></h2>
</div>
<!-- End Post -->
<?php
if (empty($_GET['id'])) {
$_GET['id'] = "";
}
$ProductID = $_GET['id']; //the product id from the URL
$action = $_GET['action']; //the action from the URL
//if there is an product_id and that product_id doesn't exist display an error message
switch ($action) { //decide what to do
case "add":
$_SESSION['cart'][$ProductID] ++; //add one to the quantity of the product with id $product_id
break;
case "remove":
$_SESSION['cart'][$ProductID] --; //remove one from the quantity of the product with id $product_id
if ($_SESSION['cart'][$ProductID] == 0)
unset($_SESSION['cart'][$ProductID]); //if the quantity is zero, remove it completely (using the 'unset' function) - otherwise is will show zero, then -1, -2 etc when the user keeps removing items.
break;
case "empty":
unset($_SESSION['cart']); //unset the whole cart, i.e. empty the cart.
break;
}
?>
<?php
if (empty($_SESSION['cart'])) {
$_SESSION['cart'] = "";
}
if ($_SESSION['cart']) { //if the cart isn't empty
//show the cart
echo "<table border=\"1\" padding=\"3\" width=\"40%\">"; //format the cart using a HTML table
//iterate through the cart, the $product_id is the key and $quantity is the value
foreach ($_SESSION['cart'] as $ProductID => $quantity) {
//get the name, description and price from the database - this will depend on your database implementation.
//use sprintf to make sure that $product_id is inserted into the query as a number - to prevent SQL injection
if ($stmt = $conn->prepare("SELECT * FROM products WHERE ProductID=?")) {
$stmt->bind_param("i", $ProductID);
$stmt->execute();
$stmt->bind_result($ProductID, $Title, $Description, $Price, $Stock, $Image, $Category, $Status);
$stmt->fetch();
// show the form
}
$total = "";
$line_cost = $Price * $quantity; //work out the line cost
$total = $total + $line_cost; //add to the total cost
echo "<tr>";
//show this information in table cells
echo "<td align=\"center\">$Title</td>";
//along with a 'remove' link next to the quantity - which links to this page, but with an action of remove, and the id of the current product
echo "<td align=\"center\">$quantity X</td>";
echo "<td align=\"center\">$line_cost</td>";
echo "</tr>";
}
//show the total
echo "<tr>";
echo "<td colspan=\"2\" align=\"right\">Total</td>";
echo "<td align=\"right\">$total</td>";
echo "</tr>";
//show the empty cart link - which links to this page, but with an action of empty. A simple bit of javascript in the onlick event of the link asks the user for confirmation
echo "<tr>";
echo "<td colspan=\"3\" align=\"right\">Empty Cart</td>";
echo "</tr>";
echo "</table>";
} else {
//otherwise tell the user they have no items in their cart
echo "You have no items in your shopping cart.";
}
?>

Answer 1

Shot in the dark, but only solution I can see.
Do you ever create the $_SESSION['cart']-array?
Try this:
switch ($action) {
case "add":
if (!isset($_SESSION['cart'][$ProductID])) {
$_SESSION['cart'][$ProductID] = 1;
}
else {
$_SESSION['cart'][$ProductID]++;
}
break;