Skype. Or you can try something like uStream - although not sure if that's 2-way video. Building your own will probably involve flash. Like Jorge says, PHP is server-side and I can't see how this factors into you deseign.
You may be able to do something with node.js / meteor, but I've not explored it sufficiently to comment further.
PHP may not be the candidate for this type of application, and can be very limited to authencating the users.