<?php
$dom = new DOMDocument();
$dom->loadXML(<<<XML
<!DOCTYPE HTML>
<html>
<body>
<main>
<p>Hello, world!</p>
</main>
</body>
</html>
XML);
$xpath = new DOMXPath($dom);
$main = $xpath->query("//main")[0];
var_dump($dom->documentElement->contains($main));
?>
The above example will output: