how should I implement dynamic subdomains?

[Yanni Chiu]

On some sites (e.g. foobar.com, for illustrative purposes), you register 
a new user (e.g. trythis). A bunch of stuff is done behind the scenes. 
Then you end up at a brand new subdomain called trythis.foobar.com

I want to implement something like this on a Ubuntu server with Apache2. 
Does anyone have any pointers on how to go about doing it?

TIA.

-- 
Yanni
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