Problem with headers?

I’ve recently installed the latest wordpress version and now I’m having a strange problem with it.
Some people are able to view news and some visitors can’t and they get error:

Warning: Cannot modify header information – headers already sent by (output started at /home/user/public_html/directory/header.php:6) in /home/user/public_html/directory/wp-includes/pluggable.php on line 689

Maybe the problem is caused by editing the index.php file?
Since I wanted to have the header in main directory (because I’m using wordpress for news only, not for my content)

I’ve did something like that with index file:

<?php
include(‘header.php’);
?>
<?php
/* Short and sweet */
define(‘WP_USE_THEMES’, true);
require(‘./wp-blog-header.php’);
?>
<? include(‘footer.php’); ?>