Connect php: erreur

Bonjour,

<?php
session_start ();
session_unset ();
session_destroy ();
header ('location: d:/projet/index.htm');
?>

elle affiche erreur suivante:

Warning: Cannot send session cache limiter - headers already sent (output started at c:\program files\easyphp\www\connecter2\logout.php:1) in c:\program files\easyphp\www\connecter2\logout.php on line 2

Warning: Cannot add header information - headers already sent by (output started at c:\program files\easyphp\www\connecter2\logout.php:1) in c:\program files\easyphp\www\connecter2\logout.php on line 5

Pourquoi elle affiche ces deux erreurs?

merci