getting base domain and sub domains from url?

Hello all, I am trying to extract base domains and sub domains from url's, and expect there to exist something to do this already. I used the parse_url($url) to get the host variable. My thought is to use $domain_elements[]=array_reverse(explode('.',$url)); then simply check $domain_element[0] against some base of international+country specific extensions, if they have sub-extensions like "co.uk" then add a third level to the domain list , and set this up as base domain. Any levels beyond the registrable domain names would count as sub domains. What I would like to know is if there are some lists of all these base domains, or maybe some function already doing what I would like to do? In advance, thanks!