Domain masking/pointing to a directory of a web application?

I built a LAMP web application that let's users create their own profile page where they can upload their resume and portfolio. The url for a person's profile will be something like

http://jobbank.com/user/johndoe

John Doe registers the domain name http://johndoefreelancer.com and he wants it to point to http://jobbank.com/user/johndoe. Anyone who visits johndoefreelancer.com should not be aware that it's driven by http://jobbank.com/user/johndoe. This means that the browser url should persistently show addresses such as:

http://johndoefreelancer.com/aboutme (really points to jobbank.com/user/johndoe/aboutme) http://johndoefreelancer.com/portfolio (really points to jobbank.com/user/johndoe/portfolio)

Additionally, clicking on any links [a href=""] should keep you at johndoefreelancer.com instead of sending you to jobbank.com.

My question is, what is the best way to achieve this?

I'm considering: 1) Give instructions to users on how to domain forward with masking

2) Instruct users to fillout the field $homeUrl in their User Profile information, which is saved to the database

3) In my PHP code, if $homeUrl exists, replace all [a href="$_SERVER['HTTP_HOST']"] with [a href="$homeUrl"]

Is this the right approach? Is there a better way?

An alternative approach would be to tell your users to point their domains to your IP and set them up as name-based virtual hosts. The benefits are:

1. It's much easier for the user to set up then forward with masking (the latter may not even be possible depending on registrar)
2. You don't have to deal with URL rewriting in your PHP code.

This would be easier to setup if you did not have to use /user/johndoe prefix for your URLs (which you don't really need to because you can do a domain lookup in your code to determine user id), but is possible with the prefix as well - it's just that mod_rewrite setup would have to be more involved in that case (you'll need to do it per domain).