public static function isSubscribed($kshow_id, $kuser_id, $subscription_type = null)
 {
     $c = new Criteria();
     $c->add(KshowKuserPeer::KSHOW_ID, $kshow_id);
     $c->add(KshowKuserPeer::KUSER_ID, $kuser_id);
     if ($subscription_type !== null) {
         $c->add(KshowKuserPeer::SUBSCRIPTION_TYPE, $subscription_type);
     }
     return KshowKuserPeer::doSelectOne($c);
 }