```php
<?php
require_once 'config.php';

// Get cart from session or cookie
$cart = isset($_SESSION['cart']) ? $_SESSION['cart'] : (isset($_COOKIE['cart']) ? json_decode($_COOKIE['cart'], true) : []);

// Calculate total
$total = 0;
foreach($cart as $item) {
    $total += $item['price'] * $item['quantity'];
}

// Handle form submission
if($_SERVER['REQUEST_METHOD'] === 'POST') {
    $customer = [
        'name' => $_POST['name'],
        'email' => $_POST['email'],
        'address' => $_POST['address'],
        'phone' => $_POST['phone']
    ];

    // DEBUG: Output cart contents for troubleshooting
    file_put_contents('cart_debug.log', print_r($cart, true));

    // Prepare order data
    $order_data = [
        'customer' => $customer,
        'items' => $cart,
        'total' => $total + 5 // Including shipping
    ];

    // Create order (this would be a call to your orders.php API in a real app)
    $order_id = createOrder($order_data['customer'], $order_data['total'], $cart);

    // Clear cart
    unset($_SESSION['cart']);
    setcookie('cart', '', time() - 3600, '/');

    // Redirect to thank you page
    header('Location: thank_you.php?order_id=' . $order_id);
    exit;
}

// Function to create order (simplified for example)
function createOrder($customer, $total, $items) {
    global $pdo;
    // Insert order with customer_phone
    $stmt = $pdo->prepare("INSERT INTO orders (customer_name, customer_email, customer_address, customer_phone, total_amount) VALUES (?, ?, ?, ?, ?)");
    $stmt->execute([$customer['name'], $customer['email'], $customer['address'], $customer['phone'], $total]);
    $order_id = $pdo->lastInsertId();

    // Insert order items
    foreach($items as $item) {
        if (!isset($item['id']) || empty($item['id'])) {
            // Skip items without a valid product_id
            continue;
        }
        $stmt = $pdo->prepare("INSERT INTO order_items (order_id, product_id, product_name, quantity, unit_price) VALUES (?, ?, ?, ?, ?)");
        $stmt->execute([$order_id, $item['id'], $item['name'], $item['quantity'], $item['price']]);

        // Update product stock
        $stmt = $pdo->prepare("UPDATE products SET stock = stock - ? WHERE id = ?");
        $stmt->execute([$item['quantity'], $item['id']]);
    }

    return $order_id;
}
?>
```