function check_valid_user()
// see if somebody is logged in and notify them if not
{
  global $HTTP_SESSION_VARS;
  if (isset($HTTP_SESSION_VARS['valid_user']))
  {
      echo 'Logged in as '.$HTTP_SESSION_VARS['valid_user'].'.';
      echo '<br / >';
  }
  else
  {
     // they are not logged in 
     do_html_heading('Problem:');
     echo 'You are not logged in.<br />';
     do_html_url('login.php', 'Login');
     do_html_footer();
     exit;
  }  
}