different between ie 5 and netscape 4.5 ?

when using ie 5, the $PHP_AUTH_USER and $PHP_AUTH_PW is set by an earlier php file.
Upon redirecting to another page, only netscape 4.5 has these variables not set.

The following is not set for netscape 4.5
<?php
if(!isset($PHP_AUTH_USER) | | !isset($PHP_AUTH_PW) ) {
Echo "Invalid Login or Login Expired !";
Echo "Try Again ! <A href='login.php'>Login Here </a>";