public static function sortOrder($attribute = null)
    {
        if (!empty($attribute)) {
            $sort = Yii::$app->request->get('sort');
            if ($sort == $attribute) {
                return ' ' . Html::tag('span', '', ['class' => 'glyphicon glyphicon-sort-by-alphabet']);
            }
            if ($sort == '-' . $attribute) {
                return ' ' . Html::tag('span', '', ['class' => 'glyphicon glyphicon-sort-by-alphabet-alt']);
            }
        }

        return null;
    }