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!
Analyze existing systems similar to the one you want to create and then figure out how each feature works in a PHP sence and then use the functions PHP provides to create the tool however you like. =)
This forum's messaging system is a good example to base your work on.