Header("location: --- not supported on PHP 4.4?

I use this script on my login page as a redirection on PHP 5 and it works:

header("Location: index.php");

but when I upload it on PHP 4.4 it says:

Warning: Cannot add header information - headers already sent by (output started at /login.php:3) in /login.php on line 10

anyone?

Thanks

Again some simple googling can find the problem for you. Usually has to do with starting sessions.

http://www.blingo.com/search?q=%22headers%20already%20sent%22&x=0&y=0