Shoutbox

in a.php

code:

---

<?
include('error.php');
// or insert your own error page in here
?>

---

in b.php

code:

---

<?
include('thepageyouwantthemtogoto.php');
?>

---

Or do you mean when they access a.php from http://my_server.com/a.php its an error but when it is accessed from http://my_server.com/b.php it runs the correct a.php page, if so you might wanna try $_SERVER['HTTP_REFERER']