[Replace tags with HTML Purifier] http://htmlpurifier.org/ #dom

index.php

<?php

// include HTMLPurifier.php

$config = HTMLPurifier_Config::createDefault();
$config->set('AutoFormat.RemoveEmpty', true);
$config->set('AutoFormat.AutoParagraph', true);
$config->set('AutoFormat.RemoveEmpty.RemoveNbsp', true);
$config->set('CSS.AllowedProperties', array());
$config->set('HTML.ForbiddenAttributes', array('*@class'));

// tag transformation
$def = $config->getHTMLDefinition(true);
$def->info_tag_transform['h2'] = new HTMLPurifier_TagTransform_Simple('strong');

$purifier = new HTMLPurifier($config);
$content = $purifier->purify($content);