Check if variable empty

Question

Code:

if ($user_id == NULL || $user_name == NULL || $user_logged == NULL) {
    $user_id = '-1';
    $user_name = NULL;
    $user_logged = NULL;
}
if ($user_admin == NULL) {
    $user_admin = NULL;
}

1. Is there any shortest way to do it?
2. And if I am right, it should be tested with is_null?
3. Is It possible $user_id, $user_name and $user_logged to write in one line (maybe array?) without repeating NULL?