The "Function session_is_registered() is deprecated" warning means just that: the function is deprecated, so you should not use it as it may be removed in a future version of PHP, as in fact has been removed in PHP 5.4. The solution is to replace it with isset($_SESSION['id']).
The "headers already sent" probably has to do with the previous warning, so fix that and this may well be fixed. If not, usually the approach is to check that you don't have whitespace at the top of your script and are not printing anything before the session has been established.