Ok, here you go.
In viewtopic.php, find:
else
$post_actions[] = '<li class="postreport"><a href="misc.php?report='.$cur_post['id'].'">'.$lang_topic['Report'].'</a>'.$lang_topic['Link separator'].'</li><li class="postdelete"><a href="delete.php?id='.$cur_post['id'].'">'.$lang_topic['Delete'].'</a>'.$lang_topic['Link separator'].'</li><li class="postedit"><a href="edit.php?id='.$cur_post['id'].'">'.$lang_topic['Edit'].'</a>'.$lang_topic['Link separator'].'</li><li class="postquote"><a href="post.php?tid='.$id.'&qid='.$cur_post['id'].'">'.$lang_topic['Quote'].'</a>';
And replace with:
else
$post_actions[] = '<li class="postreport"><a href="misc.php?report='.$cur_post['id'].'">'.$lang_topic['Report'].'</a>'.$lang_topic['Link separator'].'</li><li class="postdelete"><a href="delete.php?id='.$cur_post['id'].'">'.$lang_topic['Delete'].'</a>'.$lang_topic['Link separator'].'</li><li class="postedit"><a href="edit.php?id='.$cur_post['id'].'">'.$lang_topic['Edit'].'</a>'.$lang_topic['Link separator'].'</li><li class="postquote"><a href="post.php?tid='.$id.'&qid='.$cur_post['id'].'">'.$lang_topic['Quote'].'</a></li><br /><li class="postedit"><a href="misc.php?user='.$cur_topic['poster_id'].'&tid='.$id.'">Subscribe this poster</a></li>';
Then, open misc.php and find:
else if (isset($_GET['subscribe']))
And, add before:
else if (isset($_GET['subscribeuser']))
{
$user = intval($_GET['subscribeuser']);
$topic_id = intval($_GET['tid']);
$result=$db->query('SELECT * FROM '.$db->prefix.'subscriptions WHERE user_id='.$user.' AND topic_id='.$topic_id);
$already=$db->result($result);
if($already)
redirect($_SERVER['REQUEST_URI'], 'User is already subscribed. Redirecting...');
else
{
$result=$db->query('INSERT INTO '.$db->prefix.'subscriptions (user_id, topic_id) VALUES('.$user.' ,'.$topic_id.')') or error('Unable to add subscription', __FILE__, __LINE__, $db->error());
redirect($_SERVER['REQUEST_URI'], 'Subscribing user. Redirecting...');
}
}
So, that should work now. I already made some changes in the code in viewtopic.php, so please change that in case you already tried.