Gazelle/sections/userhistory/thread_subscribe.php

<?
// perform the back end of subscribing to topics
authorize();

if (!empty($LoggedUser['DisableForums'])) {
	error(403);
}

if (!is_number($_GET['topicid'])) {
	error(0);
}

$TopicID = (int)$_GET['topicid'];

$DB->query("
	SELECT f.ID
	FROM forums_topics AS t
		JOIN forums AS f ON f.ID = t.ForumID
	WHERE t.ID = $TopicID");
list($ForumID) = $DB->next_record();
if (!Forums::check_forumperm($ForumID)) {
	die();
}

Subscriptions::subscribe($_GET['topicid']);