I was adding some features in one of my older website for a client and saw this warning:
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.
You can simply resolve this and make this warning disappear by avoiding using the same names for session variables and normal ones.
In my case, I just had to change the following code to:
$_SESSION['type'] = $type;
To this one:
$_SESSION['category_type'] = $type;