Note that textContent property of a comment is not entity decoded, it is decoded in DOMElement::textContent :
<?php
$xml = new DOMDocument();
$xml->loadXML('<?xml version="1.0" encoding="utf-8"?>
<configuration version="2">
<!-- test --> -->
test -->
</configuration>'
);
$xpath = new DOMXPath($xml);
$comments = $xpath->query('//comment()');
$elements = $xpath->query('//configuration');
echo $comments[0]->textContent;
// result: test -->
echo $elements[0]->textContent;
// result: test -->
?>