无法弄清楚如何确定SimpleXML节点是否为空

So I have this code:

所以我有这个代码:

foreach ($xml->PrintQuestion as $PrintQuestion) {

     //if hint doesn't exist, store question id
     if (!$PrintQuestion->content->multichoice->feedback->hint->Passage) {

         fwrite($fp, "<contentid filename=\"$value\">" . $PrintQuestion->attributes()->id . "</contentid>");

     }

}
foreach ($xm