Question:

Input is a URL like this <a href="http://ws.geonames.org/children?geonameId=6255148" rel="nofollow">http://ws.geonames.org/children?geonameId=6255148</a> and I would like to receive the XML file in a SimpleXMLElement for example?

Try this as your base point:

<?php $xml = simplexml_load_file("http://ws.geonames.org/children?geonameId=6255148"); //print_r($xml); foreach($xml->geoname as $geo) { echo $geo->toponymName . "<br />"; } ?>