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 :

session_name("put_a_session_name");
session_start([
  "cookie_domain" => ".example.com",
  "cookie_path" => "/"
]);