Warning: session_start() [function.session-start]: Cannot send session cache limiter

[jyaseen]

<?php
session_start();
if( !isset($_SESSION['user_name']) or ($_SESSION['user_id'] <=0)){
	header("location:./admin_login.php");
}
?>

above is my codeing but it give me the error : Warning: session_start() [function.sessio n-start]: Cannot send session cache limiter - headers already sent (output started at C:\xampp\htdocs \RAHMAIL_2\rahm a4all\rahma_upl oad\admin_login .php:1) in C:\xampp\htdocs \RAHMAIL_2\rahm a4all\rahma_upl oad\admin_login .php on line 10

i checked for this error whole day in google. but everybody says there will be a space somewhere. but it is not.
I try this in WAMP it is working - local
but it is not working in XAMPP and in my website

[Markus]

Please see this unfinished article on the subject, namely the part on BOM.