public function get_profile_fields() {
        global $DB;
        if (!$fields = $DB->get_records('user_info_field', null, 'shortname', 'id,shortname')) {
            return null;
        }
        $res = array(0 => get_string('anyfield', 'filters'));
        foreach ($fields as $k => $v) {
            $res[$k] = $v->shortname;
        }
        return $res;
    }