if ($_POST['order_edit']){
    $order = array();

    $order['customer'] = $_POST['customerId'];
    $order['product'] = $_POST['productId'];

    add_order($order);
}

function add_order($order){
        $query = "INSERT INTO Orders (ID, ProductID, CustomerID) VALUES (NULL, '{$order['product']}', '{$order['customer']}');";
        execute_query($query);
}