WC_Order_Item_Tax::set_rate PHP Method

    public function set_rate($tax_rate_id)
    {
        $this->set_rate_id($tax_rate_id);
        $this->set_rate_code(WC_Tax::get_rate_code($tax_rate_id));
        $this->set_label(WC_Tax::get_rate_code($tax_rate_id));
        $this->set_compound(WC_Tax::get_rate_code($tax_rate_id));
    }

 /**
  * Add a tax row to the order.
  * @param array $args
  * @param int $tax_amount amount of tax.
  * @param int $shipping_tax_amount shipping amount.
  * @return int order item ID
  */
 public function add_tax($tax_rate_id, $tax_amount = 0, $shipping_tax_amount = 0)
 {
     _deprecated_function('WC_Order::add_tax', '2.7', 'Create new WC_Order_Item_Tax object and add to order with WC_Order::add_item()');
     $item = new WC_Order_Item_Tax(array('rate_id' => $tax_rate_id, 'tax_total' => $tax_amount, 'shipping_tax_total' => $shipping_tax_amount));
     $item->set_rate($tax_rate_id);
     $item->set_order_id($this->get_id());
     $item->save();
     $this->add_item($item);
     wc_do_deprecated_action('woocommerce_order_add_tax', array($this->get_id(), $item->get_id(), $tax_rate_id, $tax_amount, $shipping_tax_amount), '2.7', 'Use woocommerce_new_order_item action instead.');
     return $item->get_id();
 }