In soccer penalty kicks if the player kicks the ball before the referee blows the whisle what happens?

It all depends on the referee on the day. One thing for certain is, the penalty must be retaken, and the player who took the penalty may get a yellow card. It is however very unlikely a player will take a penalty before the whistle. It is more common in free kicks. If a free kick is taken before the whistle is blown the free kick must be retaken and again the player may get a yellow card. You will notice before a free kick or penalty the referee will hold up his whistle and show it to the players and say wait for my whistle.