Re php4.3.4

Re previous problem
Thanks to Alvaro, I solved the original problem on php version 4.3.4
However, I now have another one.

I have an admin panel which requires a password to access it from a previous
page. With the site live on an ISP server, everything is OK.
On my local server running Apache and php 4.3.4, when the correct p/w is
entered on the previous page, instead of going to the correct page (which
consists of 2 frames, it simply reproduces the previous page in both frames.

The script being used is:
#-----REDIRECT AUTH USER----------
if($HTTP_SESSION_VARS['sessadmin']){
Header('Location:admin.index.php');
}
#------AUTHENTICATE PASSWORD-------
if($HTTP_POST_VARS['pass']){
if(crypt($HTTP_POST_VARS['pass'],
$HTTP_POST_VARS['pass'])=='wiOgiun64DE9.'){
session_register('sessadmin');
$sessadmin='set';
$done=1;
}

Because it works OK on an ISP, I assume it must be a setting in my local
apache/php set up.

Any help much appreciated.

Geoff.