if ( (!isset($PHP_AUTH_USER)) || ! (($PHP_AUTH_USER == $LOGIN) && ( $PHP_AUTH_PW == "$PASSWORD" )) ) { 
    header("WWW-Authenticate: Basic entrer=\"Réserver à l'Administration\""); 
    header("HTTP/1.0 401 Unauthorized"); 
    error("Accès interdit espace réservé a L'Administration !..."); 
}
else
{
blablabla
}

Warning: Cannot add header information - headers already sent by (output
started at xxxxxxxxxxxxxxxxxxxx\index.php:1) in xxxxxxxxxxxxxxxxxxxxxxx\admin.php on line 23

Warning: Cannot add header information - headers already sent by (output
started at xxxxxxxxxxxxxxxxxxxx\index.php:1) in xxxxxxxxxxxxxxxxxxxxxxx\admin.php on line 24