// is_logged_in() contains all the logic for determining if a
// request should be considered a "logged in" request or not.
// It is the core of require_login() but it can also be called
// on its own in other contexts (e.g. display one link if an admin
// is logged in and display another link if they are not)
function is_logged_in() {
  // Having a admin_id in the session serves a dual-purpose:
  // - Its presence indicates the admin is logged in.
  // - Its value tells which admin for looking up their record.
  return isset($_SESSION['admin_id']);
}

// Call require_login() at the top of any page which needs to
// require a valid login before granting acccess to the page.
function require_login() {
  if(!is_logged_in()) {
    redirect_to(url_for('/staff/login.php'));
  } else {
    // Do nothing, let the rest of the page proceed
  }
}