[PHP] Session Variables Not Being Passed

[Jami]

The code that I have is as such:

//header.php
if(isset($_POST['login'])){
  $checkuser = mysql_query("SELECT * FROM mainacct WHERE username = 
'{$_POST['username']}' AND password = '{$_POST['password']}' ", $connection) or die 
("Couldn't find user.");
  if (mysql_num_rows($checkuser) > 0){
   while ($row = mysql_fetch_array($checkuser)){
    $_SESSION['UN'] = $row['username'];
    $_SESSION['UserLoggedIn'] = True;
   } 
  } else {
      header("Location:login.php");
   exit;
  }
}

This file is then used as an include in main.php. As long as the variables are used 
ONLY in the header.php file, it works. But when I try to call the session variables in 
main.php, it doesn't show the values of UN or UserLoggedIn. session_start() is called 
in main.php. Is this a quirk with sessions, or is there something more I am supposed 
to be doing? Running Apache 2 w/PHP 4.2.3 on WindowsXP.

Jami