echo "
<h2>Alle Foren</h2>"; 
$sitemap_forum = mysql_query("SELECT forum_id, forum_name, forum_desc FROM phpbb_forums ORDER BY forum_name
WHERE forum_id = 3 OR WHERE forum_id <> 8 OR WHERE forum_id <> 29 OR WHERE forum_id <> 9 OR WHERE forum_id <> 10 OR WHERE forum_id <> 26 OR WHERE forum_id <> 28 OR WHERE forum_id <> 18 OR WHERE forum_id <> 17 OR WHERE forum_id <> 12 OR WHERE forum_id <> 11 OR WHERE forum_id <> 20 OR WHERE forum_id <> 13 OR WHERE forum_id <> 27 OR WHERE forum_id <> 14 OR WHERE forum_id <> 15");
while($row=  mysql_fetch_assoc($sitemap_forum))

... WHERE forum_id = 3 OR WHERE forum_id <> 8 OR WHERE forum_id <> 29 OR ...

SELECT
    forum_id,
    forum_name,
    forum_desc
FROM
    phpbb_forums
WHERE
    forum_id = 3
    OR (
        forum_id <> 8
        AND forum_id <> 9
        AND forum_id <> 10
        AND forum_id <> 11
        AND forum_id <> 12
        AND forum_id <> 13
        AND forum_id <> 14
        AND forum_id <> 15
        AND forum_id <> 17
        AND forum_id <> 18
        AND forum_id <> 20
        AND forum_id <> 26
        AND forum_id <> 27
        AND forum_id <> 28
        AND forum_id <> 29
    )
ORDER BY
    forum_name

SELECT
    forum_id,
    forum_name,
    forum_desc
FROM
    phpbb_forums
WHERE
    forum_id=3
    OR forum_id=8
    OR forum_id=9
    OR forum_id=10
    OR forum_id=11
    OR forum_id=12
    OR forum_id=13
    OR forum_id=14
    OR forum_id=15
    OR forum_id=17
    OR forum_id=18
    OR forum_id=20
    OR forum_id=26
    OR forum_id=27
    OR forum_id=28
    OR forum_id=29
ORDER BY
    forum_name