htmlpurifier/library/HTMLPurifier/AttrTransform/BdoDir.php

<?php

// this MUST be placed in post, as it assumes that any value in dir is valid

/**
 * Post-trasnform that ensures that bdo tags have the dir attribute set.
 */
class HTMLPurifier_AttrTransform_BdoDir extends HTMLPurifier_AttrTransform
{

    public function transform($attr, $config, $context) {
        if (isset($attr['dir'])) return $attr;
        $attr['dir'] = $config->get('Attr', 'DefaultTextDir');
        return $attr;
    }

}