php & mysql: updating two tables with one query?

[shutat]

I'm working on a basic login routine where I'd like to replace a "guest" once a member logs in, but ran into a snag with syntax.

 

Right now, I have something like below just to try things out, but it isn't really what I need.

   $res = mysql_query("update tbl_users set laston='" . $time . "' where ID='" .   $n . "' limit 1;");
             
   $res = mysql_query("update tbl_guests set laston='0' where ip='" . 
            $_SERVER["REMOTE_ADDR"] . "' limit 1;");

 

I tried various ways at an attempt for just one query, and my last try was something like

   $n = user's ID
   $time = time();
   $res = mysql_query("update tbl_users, tbl_guests, set tbl_users.laston='" .
            $time . "', tbl_users.ip='" . $_SERVER["REMOTE_ADDR"] . "' where
            tbl_users.ID='" . $n . "', set tbl_guests.laston='0' where
            tbl_guests.ip=tbl_users.ip limit 1;");

However, I got nothing but dreaded syntax errors with all attempts.  I haven't done much work at all with joins, but was wondering if it's possible to update two tables in such a way?  If so, what am I not getting right?

 

TIA