using subdomains for user profile urls using php?

Question

i was wondering how i can get users to sign up and assign them a subdomain i.e. http://username.tumblr.com like tumblr and blogger, and why do they use subdomains, and not blogger.com/username, easy urls, thanks

Answer 1

1. wildcard (*) subdomains
2. because username.service.tdl is more "individual" then service.tdl/username

for an example with symfony read Advanced-Routing

Answer 2

1. You have to create a wildcard dns entry (see this question).

2. There are a couple reason why someone might do this.

   1. It's a different domain so search engines see it as a different website and assign it its own pagerank/value.
   2. it looks "nicer" and feels more personalized to the user.