function last_modified_header() {
    $post_id = get_queried_object_id();
    if($post_id) {
        $modified_date = new DateTime();
        $modified_date->setTimestamp(get_the_modified_date('U', $post_id));
        setlocale(LC_ALL, array('en_US.UTF-8'));
        header("Last-Modified: " . $modified_date->format('D, d M Y H:i:s +0000') );
    }
}