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

Use it on every domain/subdomain:

session_name('name');
ini_set('session.cookie_domain', '.example.com');
ini_set('session.save_path', '/var/lib/php/session');
session_start();

Path for session.save_path can be different for your case but it should be the same on every domain/subdomain. It is not always true by default.