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-tutorial The only problem is I get the error mentioned on the title on this line of code:

$_SESSION['cart'][$product_id]++;

Heres the whole 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;

        }

    ?>

Ben de php ayarlarına kayıt küresellerle etkinleştirin ettik. Ben bunu düzeltmek için ne yapmalıyım?