Directory style url for replicating site???

Hi

Could you point me into the right direction for how to do a directory style urls for a multi-user self replicating website.

I currently use www.site.com?user=username

I would like to know how to have
www.site.com/username

or even more preferred
www.username.site.com

One other ingredient...
I have a tracking code where the member just alters the tracking code used each time they place advertising....
www.site.com?user=username&tracking=trackingcode. Normally written as t=1 t=s t=z

So I would like to have (where tracking code is z)

www.site.com/username/z

or www.username.site.com/z

I assume that
www.username.z.site.com is not possible

All of my programing is written for ?user=username.
How is the php/mysql different for directory style?

Thanks to all.

I don't know the logistics of it but many hosts, including the one that I own offer unlimited sub-domains.