public function validation($data, $files) {
        global $CFG;

        $errors = parent::validation($data, $files);

        require_once($CFG->libdir . '/simplepie/moodle_simplepie.php');

        $rss = new moodle_simplepie();
        $rssfile = $rss->registry->create('File', array($data['url']));
        $filetest = $rss->registry->create('Locator', array($rssfile));

        if (!$filetest->is_feed($rssfile)) {
            $errors['url'] = get_string('feedisinvalid', 'blog');
        } else {
            $rss->set_feed_url($data['url']);
            if (!$rss->init()) {
                $errors['url'] = get_string('emptyrssfeed', 'blog');
            }
        }

        return $errors;
    }