Does user_is_logged_in() check if there is at least one user logged in? Does the user_is_logged_in() method return TRUE if there is at least one user logged in? Or does it refer to the current session, and returns TRUE if the current user is logged in?
By looking at the Drupal API, you can see the function itself:

&lt;?php
function user_is_logged_in() {
  return (bool) $GLOBALS['user']-&gt;uid;
}
?&gt;


Which shows that it returns TRUE if the current session is a logged-in user, or FALSE if the current session is an anonymous user.