I came across this error a few times, enough to where I dug around and finally found the real fix to suppress the error completely.
Warning: Unknown: Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively in Unknown on line 0
I was able to suppress the error by properly upsetting a session variable:
$_SESSION[$field] = null;
The warning is triggered by assigning to the $_SESSION super global directly. By using unset(…) we are not assign null directly.