Sorry, my previous posting worked only for the top node. Here the corrected version, which will work for any node:
XML
----
<?xml version="1.0"?>
<contacts>
 <person>Adam</person>
 <person>Eva</person>
 <person>Thomas</person>
</contacts>
PHP
---
<?php
// load XML, create XPath object
$xml = new DomDocument();
$xml->preserveWhitespace = false;
$xml->load('contacts.xml');
$xpath = new DOMXPath($xml);
// get node eva, which we will append to
$eva = $xpath->query('/contacts/person[.="Eva"]')->item(0);
// create node john
$john = $xml->createElement('person', 'John');
// insert john after eva
// "in eva's parent node (=contacts) insert
// john before eva's next node"
// this also works if eva would be the last node
$eva->parentNode->insertBefore($john, $eva->nextSibling);
// show result
header('Content-Type: text/plain');
print $xml->saveXML();
?>

Result
------
<?xml version="1.0"?>
<contacts>
 <person>Adam</person>
 <person>Eva</person><person>John</person>
 <person>Thomas</person>
</contacts>