Certo!

codice:
$sql  = "SELECT phpbb_forums.forum_name, phpbb_topics.topic_title, ";
  $sql .= '       IF(phpbb_posts.poster_id = -1, IF(phpbb_posts.post_username = "", "Ospite", phpbb_posts.post_username), ';
  $sql .= "       phpbb_users.username) AS username, ";
  $sql .= "       phpbb_posts.post_time, phpbb_forums.forum_id, phpbb_topics.topic_id, phpbb_users.user_id, ";
  $sql .= "       phpbb_posts.post_id, phpbb_posts.poster_id ";
  $sql .= "FROM phpbb_forums, phpbb_topics, phpbb_users, phpbb_posts ";
  $sql .= "WHERE phpbb_posts.poster_id = phpbb_users.user_id ";
  $sql .= "      AND phpbb_posts.topic_id = phpbb_topics.topic_id ";
  $sql .= "      AND phpbb_posts.forum_id = phpbb_forums.forum_id ";
  $sql .= "      AND phpbb_forums.auth_read <> 2 ";
  $sql .= "ORDER BY phpbb_posts.post_time DESC ";
  $sql .= "LIMIT 0 , 5";
Se sei pratico di SQL ti posso postare anche lo schema del database di phpBB.