how to get full host name from a partial string in php

I have a the following URL:google.com.

I need to know if the full hostname consists www, or a function that will give me the full hostname (IE. www.google.com)

I want to get the full host-name and i cannot always add www as a prefix (IE. if i have docs.google.com, i don't want to add www because it will destroy the link).

Is there a way to know if i can add www as a prefix or not? I tried some of the PHP gethost() functions but no avail.