Learn PHP first. You don't have to become a guru at it, yet. Go through tutorials, buy a book or two, experiment. Try doing something small that you think you'd need to do for your project.
Once you're at least somewhat comfortable about using PHP, start looking into PHP web frameworks. There're MANY. Pick one. At this point it doesn't really matter which one it is. They all have similar concepts. If you learn one, it's much easier for you to learn the other ones afterwards.
You didn't say whether you'd be coding from scratch or using open source (or commercial) products/platforms/libraries. Typically these days, unless your idea is VERY unique, people tend to assemble rather than code. What that means that you pick products/platforms/libraries that do what they do best, then "glue" them together with your custom code. I'm not sure whether that applies to your idea, but I would assume so. If so, look around for products/platforms/libraries that provide most of what you want and are "battle tested", and extensible (if that's what you need). Play around with them, figure out how to use them, and if they don't work for you, take what works in them and re-implement the rest.
As for JavaScript...jQuery is VERY popular. There are others, but that does seem to be emerging as a defacto standard of sorts. Still, learn the basics of JavaScript first (if you don't know it already).
You also didn't mention what your background and expertise in databases is. You're gonna need some sort of database for this thing, almost certainly. If you don't know SQL, you need to learn that, too.
Once you actually start building your idea, be open minded (and smart about designing it) about replacing any part of the application stack, if after you become familiar with the parts you find out they really aren't that great for what you're trying to achieve.