htmlpurifier.php

<?php
//add rel="nofollow" and target="_blank" to links with htmlpurifier 4.1.1

require_once '../../library/HTMLPurifier.auto.php';
class HTMLPurifier_AttrTransform_AValidator extends HTMLPurifier_AttrTransform
{
	var $name = 'Link validation';

	function transform($attr, $config, $context) {
		$attr['target'] = '_blank';
		$attr['rel'] = 'nofollow';
		return $attr;
	}
}

$config = HTMLPurifier_Config::createDefault();

// configuration goes here:
$config->set('HTML.Doctype', 'XHTML 1.0 Transitional');
//let them use basic html
$config->set('HTML.AllowedElements', array('br', 'em', 'i', 'b', 'strong', 'strike', 'sub', 'sup', 'a'));
//href is the only one allowed
$config->set('HTML.AllowedAttributes', array('href'));

//new definition
$config->set('HTML.DefinitionID', 'blank_nofollow'); 
$config->set('HTML.DefinitionRev', 1);
//remove this when you go live and set 'Cache.SerializerPath'
$config->set('Cache.DefinitionImpl', null); 
//$config->set('Cache.SerializerPath', 'some/path/for/caching');
$def =& $config->getHTMLDefinition(true);

$a =& $def->addBlankElement('a');
$a->attr_transform_post[] = new HTMLPurifier_AttrTransform_AValidator();

$purifier = new HTMLPurifier($config);

// untrusted input HTML
$html = '<a rel="test" target="test2" href="mylink.php">Simple and short';

$pure_html = $purifier->purify($html);

echo $pure_html;