How can I generate a reference number in a certain pattern in php

I would like to generate a sequential number.

basically I have a form and on that form is a textbox for a reference number - I would like that number generated. Ideally I would like it to be in the format of 'MB.P1.A, all sequential, nothing random.

I am not sure how to go about this. whether it needs to be a database thing or text file thing.

MB is constant but the number after P will change randomly as well as the last letter.

How can I do this in php?