start page does not appear after login

My login seems to work well but after login I do not seem to be directed to the start page.  Looks like the line header("Location: ".$next_page); is not working.  I verified that $next_page has a value ("/classes/access_user/logged.php") and the script runs upto that point.  Help appreciated.

check this thread I think it's the same problem:

http://www.webdigity.com/index.php/topic,7276.0.Installation+-+need+help.html

I seem to get it to work but battling with these errors:

Warning: Cannot modify header information - headers already sent by (output started at /big/dom/xfeow/www/classes/access_user/login.php:9) in /big/dom/xfeow/www/classes/access_user/access_user_class.php on line 130

Seems login.php cannot have much above the code.  If I delete everything above <? include($_SERVER['DOCUMENT_ROOT'] then the login works fine.

it's for all php scripts never output data before you send a header ;)