ee,
I think you need to give us more information on this if you want to get help.
Do you want the PIC controller to act as a complete phase-detector counter and reference or do you want it to control a dedicated PLL IC?
I would not use a microprocessor to handle all tasks in a PLL if yoiu want any performance to speak of! You problably would end up using a frequency locked loop, FLL.
Post more details and we will be able to help!
Cheers
WD