okay... mod_rewrite won't work if you only ever allow the one URL.
You could do it via sessions, but you'd have to POST all of your page requests in order to use only one URL.
you might consider using javascript (xhr) and simply loading everything dynamically.
but what it comes down to is this: it will be highly unusable. you'll be forcing users to use whatever nav system you implement, making the forward/back buttons and address bar effectively useless.
I'd highly recommend stepping back and considering why you're doing this. If there's a good reason, maybe I could suggest a good solution. but if there's not (or, if you won't say what the reason is), then my advice might be less than useful (and/or welcome).