Facebook does this, for example. In a static page I’d do it with a folder and index file for each user, but that’s not practical if you have too many users. Could you explain how this is done or at least how it is called so I can find myself something to read?
How is your site constructed? Is it static, or does it use a CMS, or are you trying to build something in PHP yourself to automate this. How do users get registered etc? Will you do it, or do they sign up and get the URL automatically?