prestashop - add order id or order ref to email subject

Mail.php

<?php
/**
* NOTICE OF LICENSE
*
* This source file is subject to the Software License Agreement
* that is bundled with this package in the file LICENSE.txt.
* 
*  @author    Michael Hjulskov
*  @copyright 2014-2015 Michael Hjulskov
*  @license   http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0) 
*/

class Mail extends MailCore
{
	public static function Send($id_lang, $template, $subject, $template_vars, $to,
		$to_name = null, $from = null, $from_name = null, $file_attachment = null, $mode_smtp = null,
		$template_path = _PS_MAIL_DIR_, $die = false, $id_shop = null, $bcc = null)
	{
		// add order id or ref to email subject
		if (isset($template_vars['{order_name}']) && $template_vars['{order_name}'])
			$subject .= ' (ID '.$template_vars['{order_name}'].')';
		
		return parent::Send($id_lang, $template, $subject, $template_vars, $to, $to_name, $from, $from_name, $file_attachment, $mode_smtp, $template_path, $die, $id_shop, $bcc);
	}
}