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The digital pot have a end to end resistance tolerance of 20% , output voltage must be less than 6.25v ,5v (maximum voltage on the pot ) +1.25 (LM317 regulated voltage).
See the tipical use of LM317 (fist page) http://www.electro-tech-online.com/custompdfs/2009/08/LM117-4.pdf and look at the formula at the bottom of the page:
The current on the ADJ pin is somewhere between 0 and 100uA , using a 10k digital pot this will giwe an error of R2*Iadj = 10000*0,0001 = 1V , so you have to use for R2 a smaller value than a digital pot can have. If you don't need a very good precision you better use a resistive net connected to the portb this will give you in the worst case an error of 0.6 V
1 - The Rds of 2N5461 is temperature dependent so the output voltage will be temperature dependent
2 - The PWM output is referenced to PIC supply voltage
If this is a school project I think the idea is to use the precision and the temperature stability of LM317 to build a digital controlled voltage source , both ideas have the same lack , if you multiply (first case) or add (the second case) an imprecise value to a precise value you will have an imprecise value at the end. Sorry for the criticism , I'm still searching for an working solution.
Improved version of digital pot solution, the Iadj will go through the op amp repeater output , not through R1 R2 divider
You will need a calibration procedure because the digital pot have a tolerace of 20%.