Where is the update thread time, poster etc?

I'm trying to implement some primitive form of thread cross posting and think I've worked out how to do it for two threads.

Basically I create a standard pointer thread which points at the real thread, however I add an extra field to the thread table so the real thread can point back at the threadid of the pointer.

So then all I need to do when someone posts a reply is to update the lastpost time, poster etc on the pointer.

Can anyone point me to the routine that updates the thread info so I can modify it to update the other threads information also?

Thanks,
Mark

[Admin]

In newreply.php:

        if ($threadinfo[replycount]%10==0) {
          $replies=$DB_site->query_first("SELECT COUNT(*)-1 AS replies FROM post WHERE threadid='$threadid'");
          $DB_site->query("UPDATE thread SET lastpost='".time()."',replycount='$replies[replies]',lastposter='".addslashes($postusername)."' WHERE threadid='$threadid'");
        } else {
          $DB_site->query("UPDATE thread SET lastpost='".time()."',replycount=replycount+1,lastposter='".addslashes($postusername)."' WHERE threadid='$threadid'");
        }