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 -->
?>