<?php
$list = '<ul>
<li class="woman">photo</li>
<li class="woman">photo</li>
<li class="woman">photo</li>
<li class="woman">photo</li>
<li class="woman">photo</li>
<li class="woman">photo</li>
<li class="woman">photo</li>
<li class="woman">photo</li>
<li class="woman">photo</li>
</ul>';
$dom = new DOMDocument;
$dom->preserveWhiteSpace = false;
$dom->loadXml($list);
$dom->documentElement->lastChild->setAttribute('class', 'man');
$dom->formatOutput = true;
echo $dom->saveXml($dom->documentElement);