If you have a Drupal 6 site, and need to secure a page so only users logged in can see it, do the following…

if(user_is_logged_in() != TRUE){
   // Do stuff, i.e.
   // header('Location: /');
   // exit('You do not have permission to view this page, please log in with the correct access.');
}

This makes use of the Drupal 6 function user_is_logged_in()

This can also be used in the page.tpl with a page title, (which uses the $title variable)…

if($title == 'Secured Page' && user_is_logged_in() == TRUE){
   // Do stuff that will only show if the page title is Secured Page and the user is logged in.
}

This can also be used to specify just the administrator, using the $is_admin variable…

if($is_admin != TRUE) {
   // Redirect or exit
   // header('Location: /');
   // exit('You do not have permission to view this page, please log in with the correct access.');
}