Help with query for private message

[DRJ]

I am trying to query the db for a value and have it displayed in the user/postbit section of a PM. I was able to do this for the Showthread, but not for PM.

I tried using this hook:

Hook Location : postbit_display_start

PHP Code:

  $kbcount = $db->query("SELECT * 
FROM test_vba_kbentries
WHERE coder_id = $post[userid] 
AND kb_approved = 1");
$post['kbentries'] = $db->num_rows($kbcount); 


But I get this error:

Fatal error: Call to a member function on a non-object in /hsphere/local/home/vbaexpre/vbaexpress.com/forum/includes/class_postbit.php(251) : eval()'d code on line 14

Is $db not valid?

Thanks