[PHP] Object Undefinded index

[Steve Turner]

I am just learning to use objects. I am messing around with a shopping cart
object, but am having confusing problems. I am getting an undefined index
error when trying to assign a key value pair to an array variable within an
object. I am confused becuase I believe this is how you assign array
variables outside an object. It acts like the variable is undefined. Thanks
for the help.

Steve Turner

//--------------------------------------------------
class Cart
{
    var $items;  // Items in shopping cart

    // Add $quantity articles of $product_id to the cart

    function add_item ($product_id, $quantity = 1)
    {
        $this->items[$product_id] += $quantity;
    }

    // Take $quantity articles of $product_id out of the cart

    function remove_item ($product_id, $quantity)
    {
        if ($this->items[$product_id] > $quantity)
  {
            $this->items[$product_id] -= $quantity;
            return true;
        }
  else
  {
            return false;
        }
    }
}

//I am interacting with the object with the following code
$cart = new Cart;
$cart->add_item("10", 1);

foreach ($cart as $product_id => $quantity)
{
echo $product_id . "<br>" . $quantity;
}

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