I remember that PHDC was supported in the very old MLA stack (I would say proably in the MAL from june 2013 (look under the legacy tab) : https://www.microchip.com/mplab/microchip-libraries-for-applications
BUT, please pay attention that at that time PIC32MM did not exist and also that the compilers were slightly different.
PIC32MX is nbot the same core that PIC32MM but for the USB device at C level you should beable to find something (maybe it was supported only for PIC24F at that time...)
Make sure to go to USB.org website. The specification from PHDC class should be available