NotificationTarget::getMessageID PHP Method

    function getMessageID()
    {
        return "";
    }

 /**
  * @param $target              NotificationTarget object
  * @param $tid          string template computed id
  * @param $user_infos   array
  * @param $options      array
  **/
 function getDataToSend(NotificationTarget $target, $tid, array $user_infos, array $options)
 {
     $language = $user_infos['language'];
     $user_email = $user_infos['email'];
     $user_name = $user_infos['username'];
     $sender = $target->getSender($options);
     $replyto = $target->getReplyTo($options);
     $mailing_options['to'] = $user_email;
     $mailing_options['toname'] = $user_name;
     $mailing_options['from'] = $sender['email'];
     $mailing_options['fromname'] = $sender['name'];
     $mailing_options['replyto'] = $replyto['email'];
     $mailing_options['replytoname'] = $replyto['name'];
     $mailing_options['messageid'] = $target->getMessageID();
     $template_data = $this->templates_by_languages[$tid];
     $mailing_options['subject'] = $template_data['subject'];
     $mailing_options['content_html'] = $template_data['content_html'];
     $mailing_options['content_text'] = $template_data['content_text'];
     $mailing_options['items_id'] = $target->obj->getField('id');
     if (isset($target->obj->documents)) {
         $mailing_options['documents'] = $target->obj->documents;
     }
     return $mailing_options;
 }