如何用domxml实现把节点和数据插入到已有的xml文档中

Question

  $content = new DomDocument(); 
    $content->load('shuxue.xml'); 
    $playlist = $content->documentElement; 
    $tracklist1 = $playlist->documentElement;
   // $tracklist = $content->createElement('tracklist'); 
   // $playlist->appendChild($tracklist); 
    $track = $content->createElement('track');
    $tracklist1->appendChild($track);
    $title = $content->createElement('title'); 
    $title->appendChild($content->createTextNode($_POST['title'])); 
    $track->appendChild($title);
    $song_id = $content->createElement('song_id'); 
    $song_id->appendChild($content->createTextNode($_POST['title'])); 
    $track->appendChild($song_id);
    $fp = fopen("shuxue.xml","w");
    $info = $content->savexml();
      if(fwrite($fp,$info))
      echo "success";
      else 
      echo "fail";