معنى علامة "؟" في أكواد php

// update product quantities
foreach ($products as $product) {
    $stmt = $conn->prepare('UPDATE products SET quantity = quantity - ? WHERE id = ?');
    $stmt->bind_param('ii', $product['quantity'], $product['id']);
    $stmt->execute();
    $stmt = $conn->prepare('INSERT INTO order_items (order_id, product_id, quantity, price) VALUES (?, ?, ?, ?)');