yii\web\Request::getETags PHP Method

    public function getETags()
    {
        if (isset($_SERVER['HTTP_IF_NONE_MATCH'])) {
            return preg_split('/[\\s,]+/', str_replace('-gzip', '', $_SERVER['HTTP_IF_NONE_MATCH']), -1, PREG_SPLIT_NO_EMPTY);
        } else {
            return [];
        }
    }