Drupal通知模块-如何获取订阅ID(SID)?

// find subscription
$subs = notifications_user_get_subscriptions(
        $user->uid,
        'node',
        $topic->nid, 
        $topic,     
        FALSE);

// compose link
$destination = "?destination=forum/idea-exchange";
if ($subs) {
   foreach ($subs as $key => $sub) {
      $link = notifications_get_link('unsubscribe', array(
              'sid' => $sub->sid, 
              'confirm' => FALSE));
      $variables['topics'][$id]->subscribe_link = 
         '<a class="unsubscribe" href="/'.$link['href'].
               $destination.'">'.t('Stop tracking this topic').'</a>';
      break;
   }
}
else {
  $link = notifications_get_link(
        'subscribe', 
        array('uid' => $user->uid, 
              'type' => 'thread', 
              'fields' => array('nid' => $topic->nid), 
              'confirm' => FALSE));
  $variables['topics'][$id]->subscribe_link = 
      '<a class="subscribe" href="/'.
       $link['href'].$destination.'">'.t('Track this topic').'</a>';
}

this