Description:
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PHP does not accept alternative cookie domains set through either session_set_cookie_params or ini_set.

My test machine runs two sub domains 'www.example.com' and 'alt.example.com'. In my script I have tried to set the cookie domain to '.example.com' (I've also tried 'example.com') so that both domains can access the cookie. When the cookie is sent to the browser the domain is set as the current domain ('www.example.com' or 'alt.example.com') rather than what I specified.

I get the expected behaviour when I set session.cookie_domain in php.ini or when I manually set a cookie.

Reproduce code:
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<?php
session_set_cookie_params(6 * 3600, '/', '.example.com');
session_start();
print_r(session_get_cookie_params()); //Shows correct values
echo ini_get('session.cookie_domain') . '<br />'; //Outputs '.example.com'
echo $_SESSION['blah'] . '<br />';
$_SESSION['blah'] = rand();
?>

php.ini:

session.use_cookies = 1
session.name = PHPSESSID
session.auto_start = 1
session.cookie_lifetime = 0
session.cookie_path = /
session.cookie_domain = 
session.serialize_handler = php
session.cache_expire = 180
session.use_trans_sid = 0
session.hash_function = 0
session.hash_bits_per_character = 5

Expected result:
----------------
Cookie domain (in client): example.com

Actual result:
--------------
Cookie domain (in client): www.example.com