<?php
// Add an item to the cart
function add_item(array $cart, string $key, int $quantity, array $product) {
    if ($quantity > 0) {
        // If item already exists in cart, update quantity
        if (isset($cart[$key])) {
            $quantity += $cart[$key]['qty'];
            update_item($key, $quantity);
            return;
        } else {
            // Add item
            $item = array(
                'name' => $product['name'],
                'cost' => $product['cost'],
                'qty'  => $quantity,
                'total' => $product['cost'] * $quantity,
            );
            $cart[$key] = $item;
        }
    }
    return $cart;
}

// Update an item in the cart
$cart = [];
function update_item(array $cart, string $key, int $quantity) {
    if (isset($cart[$key])) {
        if ($quantity <= 0) {
            unset($cart[$key]);
        } else {
            $cart[$key]['qty'] = $quantity;
            $total = $cart[$key]['cost'] *
                     $cart[$key]['qty'];
            $cart[$key]['total'] = $total;
        }
    }
    return $cart;
}

// Get cart subtotal
function get_subtotal(array $cart) {
    $subtotal = 0;
    foreach ($cart as $item) {
        $subtotal += $item['total'];
    }
    $subtotal_f = number_format($subtotal, 2);
    return $subtotal_f;
}
?>