Warning: Cannot add header information - headers already sent by

Hey thanks for a great tutorial!
I'm having problems with the headers though.
The error/warning...

Warning: Cannot add header information - headers already sent by (output started at c:\phpdev\www\newshopper.php:8) in c:\phpdev\www\newshopper.php on line 21


keeps on resulting when I try using the

header('Location: login.php');
and
session_start();

The functions work when they are by themselves, but when I place them in my code, I get the warning. The data is still getting to mySQL (no problems there). Here is the code I'm using. ANY help would be greatly appreciated.

<?php
$db = mysql_connect('localhost', '') or die("Couldn't connect to the database.");
mysql_select_db('') or die("Couldn't select the database");

if (isset ($submitButton)){
$_POST['userName'] = addslashes($_POST['userName']);
$_POST['password'] = md5($_POST['password']);
$sql = "INSERT into user (userName, password, name) values ('$_POST[userName]', '$_POST[password]', '$name')";
if (mysql_query($sql)){
// Start the login session
session_start();

// We've already added slashes and MD5'd the password
$_SESSION['user'] = $_POST['userName'];
$_SESSION['pass'] = $_POST['password'];

header("Location: shoppingCart.php");
die();
}
else {
echo("error ". mysql_error());
}
}
?>

To be able to use in the same script session_start() and header(),
you need to add a line :

session_cache_limiter('public');

Then you can do

session_start();

and header("location : whatever.php");


You can get some more information on the PHP site at:
http://www.php.net/manual/en/function.header.php

ciao!

warning: Cannot add header information - headers already sent in d:\web\hasp.php on line 12."
and i can't see any content.

Soultion :-

activate the flag output_buffering in the php.ini.
Step 1:-Search PHP.ini
Step 2: Find output_buffering variable
Step 3: Replace off with 4096

For instance:

output_buffering = 4096

Thanks n Regards,
Shahzad Arain
92-334-5307738