1.why stable carbocation is more reactive.

2.Is phenyl electon donating or withdrawing?

1) The reactivity of a compound depends upon the stability of it's reactive intermediate.Thus if the intermediate is carbocation then the formation of product or reactivity depends upon it's stability.
The more stable the intermediate i.e carbocation, more reactive the organic compound will be.
2) Phenyl group is electron donating group that is it has +I effect due to resonating electrons. But the presence of electron withdrawing group makes the phenyl group electron withdrawing.