PHP programming forum. Ask questions or help people concerning PHP code. Don't understand a function? Need help implementing a class? Don't understand a class? Here is where to ask. Remember to do your homework!
PhpBB is a good forum. Two others you might want to look at are Invision Board (http://www.invisionboard.com) and YaBB (http://www.yabb.info). I've tried all 3 of these and like them all. So the choice is up to you.