PHP : variable de session

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /...chemin.../fichier.php:1) in /chemin/fichier.php on line 2

Warning: Unknown: Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively in Unknown on line 0

<?php
session_start();
session_register("prenom");
session_register("nom");
$prenom = "Jean";
$nom = "Valjean";

?>

<?php
session_start();
echo "ID de session : ".session_id()."<br>";
echo "$prenom $nom";
session_unset();
session_destroy();
?>

@session_start(); 
ini_set ('session.bug_compat_42', 0);  
ini_set ('session.bug_compat_warn', 0); 
session_register("prenom");  
session_register("nom");  
$prenom = "Jean";  
$nom = "Valjean";  
 

foreach($_SESSION as $cle => $valeur)
{
echo "  <li><strong>".ucfirst($cle)." : </strong><em>".$valeur."</em></li>\n";
}