[Radzio125]

Yes! It works! Thank you very much! 🙂

[Radzio125]

Thanks for answer. Unfortunately I want to get only user ID number. I want use this to do something like this:

$last_author = id_of_the_forum_topic_last_active_user
$user = get_user_by('login',$last_author);
if (is_user_online($user->ID)) {
} else {
}

What do I need to put in place of the id_of_the_forum_topic_last_active_user?