I use XPATH and SimpleXML to change my file.
A little example...
The xml file:
<?xml version="1.0"?>
<forum uri="http://myforum.org/index.php">
<item id="1">
<title>First Post!!!</title>
<link>http://myforum.org/index.php/m/1</link>
<description>hello I'm fabio</description>
</item>
<item id="2">
<title>Re: Second post!!!</title>
<link>http://myforum.org/index.php/m/2</link>
<description>2nd good message.</description>
</item>
</forum>
And PHP handler:
<?php
$forum = simplexml_load_file('forum.xml');
/* some xpath EXAMPLES */
/* catch all items in forum */
$result = $forum->xpath('/forum/item');
/* catch all links */
$result = $forum->xpath('//link');
/* search for "Re:" in title and returns the item's id */
$result = $forum->xpath('//item[contains(title, "Re:")]/@id');
/* catch > 10 length items and returns the item's title*/
$result = $forum->xpath('//item[string-length(description) > 10]/title');
$forum->item[1]->title['url'] = "http://goo.gl/"; /* this add a an attribute */
$forum->item[0]->foo = "newnode"; /* this add content */
$forum->item[0]->foo['attrib'] = 10; /* this add a another value */
$forum->addChild('element_name', 'value'); /* this is a new element /*
/* delete value */
unset($forum->item[0]);
// XML rendering
echo $forum->asXML();