Description:
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I want to use session across base domain and some subdomains.

Cookies set manually work fine, because when I use:

setcookie('test', '123456', 0, '/', '.domain.com');

I get proper value of $_COOKIE['test'] in base domain and across all subdomains (no matter if set this cookie in base or sub domain).
But when I use:

session_set_cookie_params(0, '/', '.domain.com');
session_start();

$_COOKIE['PHPSESSID'] has different value for base domain and another value for all subdomains (one value for all subdomains)

I think it's some bug in session_start() function, because setcookie() works fine and session_get_cookie_params() returns expected results (domain = '.domain.com').