What potential pitfalls should be avoided when working with multiple product IDs in a shopping cart?

// Example of properly handling multiple product IDs in a shopping cart

// Initialize an empty array to store the product IDs and quantities
$cart = [];

// Add a product to the cart
function addProductToCart($productId, $quantity) {
    global $cart;
    
    if(isset($cart[$productId])) {
        $cart[$productId] += $quantity;
    } else {
        $cart[$productId] = $quantity;
    }
}

// Remove a product from the cart
function removeProductFromCart($productId) {
    global $cart;
    
    unset($cart[$productId]);
}

// Calculate the total price of the items in the cart
function calculateTotalPrice() {
    global $cart;
    
    $totalPrice = 0;
    
    foreach($cart as $productId => $quantity) {
        // Calculate the price of the product based on the quantity
        $productPrice = getProductPrice($productId);
        $totalPrice += $productPrice * $quantity;
    }
    
    return $totalPrice;
}

// Function to get the price of a product (example function)
function getProductPrice($productId) {
    // This is just an example function, replace with your actual logic to get the product price
    return 10; // Assuming each product costs $10
}