I'm afraid that most of the Porsche keyfob threads end up with the owner having to purchase and program a new key from the dealership. This comes at a total tune of $400 - $500.
Now, there is a slight chance that reflowing the solder joints will help, but be careful. Adding heat can destroy the small circuit boards. Many people use ovens and bake them for a short amount of time at temps at around 300˚F.
Hot air from a heat gun can work, but again, be careful to not over heat the circuit board.