note 29199 rejected from function.session-start by betz

I followed all instructions that may solve my problem. But, I still got the problem. First, I wrote session_start(); and registered a 'valid_user' in one auth.php file before <html>. After that, I have one php file need use this 'valid_user'. So, I call session_start(); then try to access $valid_user value. But, I got Warning: session_start() Cannot send session cache limiter - headers already sent. Does anyone have idea? How to fix? auth.php: <? session_start(); session_register("valid_user"); <html> ..... File 2: <html> .... <? session_start(); $username=$valid_user; ..... ?>