when you create the menu using
MENU_ITEMS = ['categoryx', 'categoryx.php'];
It links to the categoryx no problems but if your page uses sessions where the sessionID is passed to the page on the addressbar (i.e http://domain.com/categoryx.php?SessionID=xxxxxxx)
Then you will lose your session upon clicking on the menu and customers will have to relogin again.
I have a php script link(URL) which returns the URL with the session key attached on the end of it.
Does this menu support me calling my PHP script in the Link element of the menu creation such as..
MENU_ITEMS = ['categoryx', link("categoryx.php")];
You should generate MENU_ITEMS dynamically with SessionID as a parameter in URLs.
... or turn on session.use_only_cookies in the php.ini and tell your registered users to accept cookies