吐血了,提个PHP写XML的问题

代码:
<?php
$im=$_POST['upPicName'];
$it=$_POST['upPicTitle'];
$pa=$_POST['upPicAlbum'];
$pt=$_POST['upPicThumb'];//echo $im.$it.$pa.$pt;$doc = new DOMDocument();
$doc ->formatOutput = true;
$doc->load('pic.xml');$project=$doc->documentElement->getElementsByTagName('medias');//$root = $doc->createElement('media');
//$doc->appendChild($root);/**添加图片的名字
$root_im = $doc->createAttribute('imgname');
$root->appendChild($root_im);$root_im_text=$doc->createTextNode($im); 
$root_im->appendChild($root_im_text);
//结束//添加标题
$root_it = $doc -> createAttribute('title');
$root->appendChild($root_it);$root_it_text=$doc->createTextNode($it);
$root_it->appendChild($root_it_text);
//结束//添加专辑名称
$root_pa=$doc->createAttribute('section');
$root->appendChild($root_pa);$root_pa_text=$doc->createTextNode($pa);
$root_pa->appendChild($root_pa_text);
//结束//是否为标题
$root_pt=$doc->createAttribute('thumb');
$root->appendChild($root_pt);$root_pt_text=$doc->createTextNode($pt[0]);
$root_pt->appendChild($root_pt_text);
**///结束$doc->saveXML('pic.xml');
?>XML:
<?xml version="1.0" encoding="utf-8"?>
<medias>
<media title="Independence Orchestra" section="01_independence" imgname="01_independence.png" thumb="yes"/>
<media title="Independence Orchestra" section="01_independence" imgname="independenceTreadmill.jpg" thumb="no"/>
<media title="Shawn Clothing" section="02_shawn" imgname="02_shawn.png" thumb="yes"/>
<media title="Shawn Clothing" section="02_shawn" imgname="ShawnTreadmill.jpg" thumb="no"/>
<media title="Foo Fighters" section="03_foofighters" imgname="03_foofighters.png" thumb="yes"/>
<media title="Foo Fighters" section="03_foofighters" imgname="FooFighterTreadmill.jpg" thumb="no"/>
</medias>错误提示:
Warning: DOMDocument::saveXML() expects parameter 1 to be DOMNode, string given in /home/dan_ma/public_html/LLad/upDateAction.php  on line 54
第54行为$doc->saveXML('pic.xml');  我把全部都注释掉后,什么都不做,直接保存,也报这个错误. 请高人解答,谢谢.