Toggles three classes, two without force, and one with.
<?php
$dom = Dom\HTMLDocument::createFromString('<p class="font-bold important"></p>', LIBXML_NOERROR);
$p = $dom->body->firstChild;
$classList = $p->classList;
$classList->toggle('font-bold', 'font-small');
$classList->toggle('important', force: true);
echo $dom->saveHtml($p);
?>
The above example will output:
<p class="font-bold important"></p>