I'm confused about that too. I haven't seen anything about the current Garter King At Arms retiring. The current one hasn't been in office a long time, I think. As for Philip, I think he was a Supernumerary Knight and not part of the 24 regular Knights, so there wouldn't be a spot to fill.
Correction: Philip was a Royal Knight of the Garter, so he fell into the same class as William, the Dukes of Gloucester and Kent, etc. According to Wikipedia there are three spots available for the regular 24.