Código PHP:
srand((double)microtime()*1000000);
$seed = rand(1000000,9999999);
srand((double)microtime()*$seed);
$yadda = rand (1,100);
$bdsql = "SELECT
topic_id,
topic_title,
topic_time,
topic_views,
username,
forum_id
FROM manual_topics, phpbb_users
WHERE forum_id = 28 AND user_id = topic_poster
ORDER BY $yadda LIMIT 0, 1";
Mejor, ¿no?