<?php
$xml = <<< XML
<?xml version="1.0" encoding="UTF-8" ?>
<RESPONSE>
	<MULTIPLE>
		<SINGLE>
			<KEY name="id">
				<VALUE>1</VALUE>
			</KEY>
			<KEY name="username">
				<VALUE>guest</VALUE>
			</KEY>
			<KEY name="firstname">
				<VALUE>Guest user</VALUE>
			</KEY>
		</SINGLE>
	</MULTIPLE>
</RESPONSE>
XML;

$dom = new DOMDocument;
$dom->loadXml($xml);
$xpath = new DOMXPath($dom);
echo $xpath->evaluate('string(//KEY[@name="username"]/VALUE)');

guest