请问php怎么在已有的xml文件上编写？

这是xml文件
XML code:
<?xml version="1.0" encoding="iso-8859-1"?>
<gallery>
<images>
<title><![CDATA[ ]]></title>
<description><![CDATA[ ]]></description>
<tmb>FphotoGallery/tmb1.jpg</tmb>
<img>FphotoGallery/img1.jpg</img>
</images>
<gallery>


怎么用php在后面添加内容？
我这么写的
PHP code:
$dom = new DOMDocument('1.0');
$gallery = $dom->gappendChild($dom->createElement('gallery'));
$images = $gallery->gappendChild($dom->createElement('images'));
$title = $images->gappendChild($dom->createElement('title'));
$title->gappendChild($dom->createTextNode('<![CDATA[ ]]>'));
$description = $images->gappendChild($dom->createElement('description'));
$description->gappendChild($dom->createTextNode('<![CDATA[ ]]>'));
$tmb = $images->gappendChild($dom->createElement('tmb'));
$tmb->gappendChild($dom->createTextNode('FphotoGallery/a.jpg'));
$img = $images->gappendChild($dom->createElement('img'));
$img->gappendChild($dom->createTextNode('FphotoGallery/a.jpg'));
$dom->formatOutput = true;
$dom -