Cannot increment/decrement overloaded objects nor string offsets in php

[cipcsl]

I'm trying to make a simple php shopping cart. I don't have any idea how to make one myself so I tried searching and found this one the simplest to understand for a beginner like me:http://jameshamilton.eu/content/simple-php-shopping-cart-tutorialThe only problem is I get the error mentioned on the title on this line of code:\[code\]$_SESSION['cart'][$product_id]++;\[/code\]Heres the whole code: \[code\] $product_id = $_GET['id']; //the product id from the URL $action = $_GET['action']; //the action from the URL //if there is an product_id and that product_id doesn't exist display an error message if($product_id && !productExists($product_id)) { die("Error. Product Doesn't Exist"); } switch($action) { case "add": $_SESSION['cart'][$product_id]++; //add one to the quantity of the product with id $product_id break; case "remove": $_SESSION['cart'][$product_id]--; //remove one from the quantity of the product with id $product_id if($_SESSION['cart'][$product_id] == 0) unset($_SESSION['cart'][$product_id]); //if the quantity is zero, remove it completely (using the 'unset' function) - otherwise is will show zero, then -1, -2 etc when the user keeps removing items. break; case "empty": unset($_SESSION['cart']); //unset the whole cart, i.e. empty the cart. break; } ?>\[/code\]I've also enable register globals on php settings. What do I do to fix this one?