find the first h2 tag inside a paragraph and insert a div after this tag

<?php
$dom = new DOMDocument();
$dom->loadHTML('<p><h2>Header</h2></p>');
$dom->getElementsByTagName('h2')->item(0)->parentNode->insertBefore($dom->createElement('div'),$dom->getElementsByTagName('h2')->item(0)->nextSibling);
echo $dom->saveHTML();
?>