$query_rs .= "SELECT * FROM biweb_news WHERE title LIKE ''%biweb%''
OR content LIKE ''%biweb%'' UNION SELECT * FROM biweb_user WHERE title LIKE
''%biweb%'' OR content LIKE ''%biweb%'' ORDER BY submit_date DESC";

使用UNION可以把多个select的结果集合并后，进行排序，可以轻松实现多表查询结果合并。