PunBB + PE

else if (isset($_GET['subscribe']))
{
    if ($pun_user['is_guest'] || $pun_config['o_subscriptions'] != '1')
        message($lang_common['No permission']);

    // Accept comma-separated id list
    $topic_ids = (strpos($_GET['subscribe'], ',') !== false) ? explode(',', $_GET['subscribe']) : array($_GET['subscribe']);
    $topic_ids = array_map('intval', $topic_ids);
    if (count($topic_ids) < 1)
        message($lang_common['Bad request']);

    // Make sure the user can view the topic
    foreach ($topic_ids as $topic_id)
    {
        $result = $db->query('SELECT 1 FROM '.$db->prefix.'topics AS t LEFT JOIN '.$db->prefix.'forum_perms AS fp ON (fp.forum_id=t.forum_id AND fp.group_id=1) WHERE (fp.read_forum IS NULL OR fp.read_forum=1) AND t.id='.$topic_id.' AND t.moved_to IS NULL') or error('Unable to fetch topic info', __FILE__, __LINE__, $db->error());
        if (!$db->num_rows($result))
            message($lang_common['Bad request']);

        $result = $db->query('SELECT topic_id FROM '.$db->prefix.'subscriptions WHERE user_id='.$pun_user['id'].' AND topic_id IN('.implode(',',$topic_ids).')') or error('Unable to fetch subscription info', __FILE__, __LINE__, $db->error());
        if ($db->num_rows($result))
            message($lang_misc['Already subscribed']);

        $db->query('INSERT INTO '.$db->prefix.'subscriptions (user_id, topic_id) VALUES('.$pun_user['id'].' ,'.$topic_id.')') or error('Unable to add subscription', __FILE__, __LINE__, $db->error());
    }

    // Try to determine if the data in HTTP_REFERER is valid (if not, we redirect to given topic after that)
    $redirect_url = (isset($_SERVER['HTTP_REFERER']) && preg_match('#^'.preg_quote($base_url).'/(.*?)\.php#i', $_SERVER['HTTP_REFERER'])) ? htmlspecialchars($_SERVER['HTTP_REFERER']) : ($base_url.'/viewtopic.php?id='.$topic_ids[0]);
    redirect($redirect_url, $lang_misc['Subscribe redirect']);
}