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 have confirmed. joreon's answer is correct. I cannot comment because my reputation is not enough so I post my comment here.

Define the constant in a config file. If you want to change it, no need to modify whole files.

define('ROOT_DOMAIN',   'mysite.example');
define('PHP_SESSION_NAME', 'MYSITE'); 

The session name can't consist of digits only, at least one letter must be present. Otherwise, a new session id is generated every time.

Use the following code to start using session

session_name(PHP_SESSION_NAME);
session_set_cookie_params(0, '/', '.' . ROOT_DOMAIN);
session_start();

I'm using this function:

function load_session() {
    if (session_status() == PHP_SESSION_NONE) {
        session_name(PHP_SESSION_NAME);
        session_set_cookie_params(0, '/', '.' . ROOT_DOMAIN);
        session_start();
    } elseif (session_name() != PHP_SESSION_NAME) {
        session_destroy();
        session_name(PHP_SESSION_NAME);
        session_set_cookie_params(0, '/', '.' . ROOT_DOMAIN);
        session_start();
    }
}
load_session(); // put it in anywhere you want to use session