cookie

I have some pages that require user authentication in order to get to the page content. I have done this by switch and case statements and MySql. everything was working fine but then I decided to set a cookie and use a user_id variable the rest of the site to identify the user. When I set the cookie with the following code: //set cookie(if not set) if (!isset($user_id)){ $token = md5(uniqid(rand())); setcookie ("user_id", $token, time()+86400, "/", ".donkey.aloha-street.com"); } I get the following error Warning: Cannot add header information - headers already sent by (output started at /home/httpd/html/test/members_j.php:8) in /home/httpd/html/test/members_j.php on line 130 But the authentication still works. Does anyone know what is happening? thanks in advance rodney I am using php4 on linux.