Redirection if a condition is met

I know this topic has been discussed a few times here and also on
php.net, but I cannot find an answer to my specific problem. I check to
see if a user is logged in before serving admin.php like so:
<?php
  session_start();
  if (!isset($_SERVER['username']) or !isset($_SERVER['password'])) {
    header("location:http://linux-place.com");
    exit;
  }
?>
These are the very first lines in the file. It works perfectly. The
problem is that I would like to output a message like "Please login
first before accessing the administration panel". How can I do that
without getting the headers already sent error?

Thanks,
Ahmed El-Daly
Received on Mon Oct 17 21:21:20 2005