php domdocument unwrap first div

<?php
$html = '<div>
  <div class="header">
    <div class="header-child">
      <h1>This is a heading</h1>
    </div>
  </div>
</div>';

$dom = new DOMDocument();
$dom->loadHTML($html);
$xpath = new DOMXPath($dom);
$divs = $xpath->query('//div');
$divs->item(0)->parentNode->removeChild($divs->item(0));
echo $dom->saveHTML();