Description:
------------
The problem described already in user contributed noted in documentation. Strange, that it is not yet fixed...
Also problem mentioned in http://bugs.php.net/bug.php?id=11213
==================================
laacz at laacz dot lv (28-Oct-2004 03:37)

If You are not using cookies to store session_id's, that does not mean, that session_start() will not send any headers at all. It still sends cache controlling information to user. For example:
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Even, if You set cache_limiter to none (via ini_set('session.use_cookies', 0) or ini_set('session.cache_limiter', 'none')), session_start() still tries to send empty headers and that causes error message "Cannot send session cache limiter - headers already sent". So, use output buffering, if You need to output something before session_start(). 
==================================
even session_cache_limiter('none') doesn't work

In my case, problem is that I call session_start(), then session_write_close() in the beginning, then send data, then call session_start(), then session_write_close() in the function, registered on shutdown.
There are two reasons, why I do so:
1) there are several processes, that access same session (I use md5($_SERVER['REMOTE_ADDR']) as session_id)
2) I can't buffer output at all. Amount of data is very large, it is a file being downloaded. It must be sent ASAP.

Reproduce code:
---------------
if(!empty($_SERVER['HTTP_RANGE'])){
	$sid = md5($_SERVER['REMOTE_ADDR']);
	ini_set('session.use_cookies', 0);
	session_cache_limiter('public'); // for IE "Open" bug
	session_id($sid);
	session_start(); //this one starts normally
	//check, if this download thread is permitted or not
	if(!empty($_SESSION['file_'.$file_id]) && $_SESSION['file_'.$file_id] >= MAX_DOWNLOAD_THREADS){
		header('HTTP/1.1 403 Forbidden', true, 403);
		exit;
	}elseif(!isset($_SESSION['file_'.$file_id])) $_SESSION['file_'.$file_id] = 0;
	$_SESSION['file_'.$file_id]++;
	session_write_close();
	//close session ASAP, because else other processes would be blocked

	//this function will decrement threads counter...
	function decrementCounter($sid, $file_id)
	{
//this must help us evade "Warning:  session_start(): Cannot send session cache limiter - headers already sent"
		session_cache_limiter('none');

		session_id($sid);
		session_start(); // this is line #45
		//decrement threads counter
		$_SESSION['file_'.$file_id]--;
		session_write_close();
		//close session ASAP
	}

	//... when script finished
	register_shutdown_function('decrementCounter', $sid, $file_id);
}

Expected result:
----------------
No warnings in log file.

Actual result:
--------------
[Tue Mar 22 18:32:08 2005] [error] PHP Warning:  session_start(): Cannot send session cache limiter - headers already sent (output started at /home/httpd/pantech/htdocs/file.php:144) in /home/httpd/pantech/htdocs/file.php on line 45

reproduced always, when user downloading a file

line 144 sends data to client:
echo fread( $fp, $size > READ_BLOCK_SIZE ? READ_BLOCK_SIZE : $size );