Undefined offset for $_SESSION variable - php

Question

I have been trying to create a very simple shopping cart and I'm having a problem with my $_SESSION array. This is for a school project and I'm trying to make it as simple as possible.
The error I'm getting is:
Notice: Undefined index: cart in C:\xampp\htdocs\Final\menu.php on
line 31
Notice: Undefined offset: 5 in C:\xampp\htdocs\Final\menu.php on
line 31
if(isset($_GET['id'])){
$product_id = $_GET['id'];
$_SESSION['cart'][$product_id]++;
print_r($_SESSION);
print "<br>";
print_r($_GET);
}
Once I've added more than one item to a particular product_id, the error goes away. This is the way the tutorial I read explained to add items to the cart. Any suggestions?

Answer 1

Looks like $_SESSION['cart'] does not yet exist. Since it's going to be an array, instantiate it first with:
if(!array_key_exists('cart', $_SESSION)) $_SESSION['cart'] = array();
Since you have not yet assigned anything to $_SESSION['cart'][$product_id], you'll get this type of error when trying to increment it. You may want to try:
$_SESSION['cart'][$product_id] = (array_key_exists($product_id, $_SESSION['cart'])) ? $_SESSION['cart'][$product_id] +1 : 1;
or as an if statement:
if(array_key_exists($product_id, $_SESSION['cart'])) $_SESSION['cart'][$product_id]++;
else $_SESSION['cart'][$product_id] = 1;

Answer 2

When you do $_SESSION['cart'][$product_id]++; you are actually doing:
$_SESSION['cart'][$product_id] = $_SESSION['cart'][$product_id] + 1;
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ generates warning if they keys do not exist yet
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^ assigns without problems if they keys do not exist yet
The assignment with newly created keys is not the problem, the warning is generated by php trying to get the actual value of $_SESSION['cart'][$product_id].
To solve this you should properly initialize the variable:
$_SESSION['cart'][$product_id] = isset($_SESSION['cart'][$product_id])
? $_SESSION['cart'][$product_id]++
: 1;