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

One thing which can mysteriously prevent session data being read on a subdomain, despite cookies being correctly set to .example.com is the PHP Suhosin patch. You can have everything configured correctly, as per the examples in the question, and it can just not work.

Turn the following Suhosin session settings off, and you're back in business:

suhosin.session.cryptua = Off 
suhosin.session.cryptdocroot = Off