<?php
$xml = <<<XML
<html>
<body>
<div class="a">
<div class="b">example</div>
</div>
<div class="c">
<div>
<div class="b">example</div>
</div>
</div>
</body>
</html>
XML;
$dom = new DomDocument();
$dom->loadHTML($xml);
$xpath = new DomXpath($dom);
var_dump($xpath->query("//*[@class='c']//*[@class='b']")->length);