He found P in pee.
He went around collecting urine from taverns all around until he had huge vats of it. He boiled it down to try and get the gold from out of the liquid. When he has it all boiled away, he looks at what he has and realizes it isn't gold, but instead what we now know as phosphorus.
That's why phosphorus is on the periodic table under "P."
A story my dad just told me.