Including Session Variables?

Hi! I’m having a small problem involving session variables and including php files. Let’s say I have two files. One has this code:

<?php
include('sessionvars.php');
echo $_SESSION["test"];
?>

The other (sessionvars.php) would be:

<?php
session_start();
header("Cache-control: private");
$_SESSION["test"] = "Test writing";
?>

and nothing would come out. Whats wrong?