htmlpurifier/library/HTMLPurifier/HTMLModule/Bdo.php

<?php

/**
 * XHTML 1.1 Bi-directional Text Module, defines elements that
 * declare directionality of content. Text Extension Module.
 */
class HTMLPurifier_HTMLModule_Bdo extends HTMLPurifier_HTMLModule
{

    public $name = 'Bdo';
    public $attr_collections = array(
        'I18N' => array('dir' => false)
    );

    public function setup($config) {
        $bdo = $this->addElement(
            'bdo', 'Inline', 'Inline', array('Core', 'Lang'),
            array(
                'dir' => 'Enum#ltr,rtl', // required
                // The Abstract Module specification has the attribute
                // inclusions wrong for bdo: bdo allows Lang
            )
        );
        $bdo->attr_transform_post['required-dir'] = new HTMLPurifier_AttrTransform_BdoDir();

        $this->attr_collections['I18N']['dir'] = 'Enum#ltr,rtl';
    }

}