Tu SOLUCION
Código PHP:
<?php
$xml='<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2" xmlns:gx="http://www.google.com/kml/ext/2.2" xmlns:kml="http://www.opengis.net/kml/2.2" xmlns:atom="http://www.w3.org/2005/Atom">
<a>
<b>Hola</b>
</a>
</kml>';
$sxe=new SimpleXMLElement($xml);
$sxe->registerXPathNamespace('ns','http://www.opengis.net/kml/2.2');
$result=$sxe->xpath('/ns:kml');
echo $result[0]->a->b;
?>
Resultado: Hola