PHP Sessions across sub domains

Question

I am trying to set up the following:

auth.example.com
sub1.example.com
sub2.example.com

If the user visits sub1.example.com or

Answer 1

I solved it like this

ini_set('session.cookie_domain', '.testdomain.example');
session_start();

Because I was working on localhost

ini_set('session.cookie_domain', '.localhost');

wasn't working, it sees .localhost as the toplevel instead of .com/.local/... (I suspect)