p>zero Celts

How was the evolution PIE *p > Celtic *zero?

p>f>h>zero?

If it's sure, why p>f and t>t k>k? Or can we deduce...

PIE *p > *f > Celtic *h >zero

PIE *t > *th > Celtic *t

PIE *k > *x > Celtic *k

And in Arabian?

Why there's no "p" in Arabian?

Joao SL

Rio