Raga, perchè mi compare questo errore adesso!?!?

Warning: Cannot modify header information - headers already sent by (output started at /web/htdocs/www.sito.it/home/sito/includes/header.php:36) in /web/htdocs/www.sito.it/home/sito/pages/exitgear.php on line 4

Il file exitgear.php è questo:
Codice PHP:
<?php
    if(isset($_SESSION['user_id'])){
        session_destroy();
        header("Location: ?page=exit");
    }
    else{
        header("Location: ?page=errexit");
    }
?>