function forum_is_subscribed($userid, $forum, $cm=null, $context=null) { //// added the two variables

    global $DB;

    if (is_numeric($forum)) {

        $forum = $DB->get_record('forum', array('id' => $forum));

    }


    if (!$cm) //// check if variable is empty

        $cm = get_coursemodule_from_instance('forum', $forum->id);


    if ($cm && !$context) //// same thing for the context

        $context = context_module::instance($cm->id);



    //// moved the $cm to the beginning to avoid unnecessary call to 'forum_is_forcesubscribed()'

    if ($cm && forum_is_forcesubscribed($forum) &&

            has_capability('mod/forum:allowforcesubscribe', context_module::instance($cm->id), $userid)) {

        return true;

    }

    return $DB->record_exists("forum_subscriptions", array("userid" => $userid, "forum" => $forum->id));

}