Thank you so much but i've gotten a error says cannot fetch so i looked up the code and instead of 'real_name' the field in mysql as 'realname' so i've changed the code and also the last part was different so i've changed the code to this
$result = $db->query('SELECT u.email, u.title, u.realname, u.url, u.location, u.use_avatar, u.signature, u.email_setting, u.num_posts, u.registered, u.admin_note, p.id, p.poster AS username, p.poster_id, p.poster_ip, p.poster_email, p.message, p.hide_smilies, p.posted, p.edited, p.edited_by, g.g_id, g.g_user_title, o.user_id AS is_online FROM '.$db->prefix.'posts AS p INNER JOIN '.$db->prefix.'users AS u ON u.id=p.poster_id INNER JOIN '.$db->prefix.'groups AS g ON g.g_id=u.group_id LEFT JOIN '.$db->prefix.'online AS o ON (o.user_id=u.id AND o.user_id!=1 AND o.idle=0) WHERE p.topic_id='.$id.' ORDER BY p.id LIMIT '.$start_from.','.$pun_user['disp_posts'], true) or error('Unable to fetch post info', __FILE__, __LINE__, $db->error());hope someone looking for this kind of info would get some help.
anyways i really appreciate your help, lie2815. thanks!