example to insert <newnode/> between <chid1/> and <child2/>

<?xml version="1.0" encoding="ISO-8859-1" ?>
<root>
<parent>
<child nr="1"/>
<child nr="2"/>
</parent>
</root>

<?php

$xml_src = 'test.xml';

// XPath-Querys
$parent_path = "//parent";
$next_path = "//parent/child[@nr='2']";

// Create a new DOM document
$dom = new DomDocument();
$dom->load($xml_src);

// Find the parent node
$xpath = new DomXPath($dom);

// Find parent node
$parent = $xpath->query($parent_path);

// new node will be inserted before this node
$next = $xpath->query($next_path);

// Create the new element
$element = $dom->createElement('newnode';

// Insert the new element
$parent->item(->insertBefore($element, $next->item();

echo $dom->saveXML();

?>