If you cast a spell this turn, you can't attack with creatures. If you attacked with creatures this turn, you can't cast spells. Whenever you roll , you gain life equal to the number of cards in your hand.
Official Rulings for Prahv
6/1/2012 : Prahv only cares about the turn-based action of declaring attackers. If an effect allows you to put a creature onto the battlefield attacking, you can cast spells later that turn.