Cookie: header not seen when expected - php

Question

Im using FireFox's HTTP Live headers to view the headers.
I wrote a script at mydomain.com that just sets a test cookie.
I thought that when we send a request to a naked URL, http://mydomain.com/script.php, cookies are sent across to all sub-domains.
But when I sent a request to http://www.mydomain.com/script.php, the cookie wasnt sent in the header request by the browser. How come ?
http://mydomain.com/script.php
GET /script.php HTTP/1.1
Host: mydomain.com
User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.8pre) Gecko/20100710 Ubuntu/9.10 (karmic) Namoroka/3.6.8pre
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
HTTP/1.1 200 OK
Server: nginx
Date: Fri, 16 Jul 2010 00:08:11 GMT
Content-Type: text/html
Transfer-Encoding: chunked
Connection: keep-alive
X-Powered-By: PHP/5.2.11
Set-Cookie: UserID=23; expires=Fri, 16-Jul-2010 01:08:11 GMT; path=/
Content-Encoding: gzip
----------------------------------------------------------
http://www.mydomain.com/script.php
GET /script.php HTTP/1.1
Host: www.mydomain.com
User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.8pre) Gecko/20100710 Ubuntu/9.10 (karmic) Namoroka/3.6.8pre
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
HTTP/1.1 200 OK
Server: nginx
Date: Fri, 16 Jul 2010 00:08:24 GMT
Content-Type: text/html
Transfer-Encoding: chunked
Connection: keep-alive
X-Powered-By: PHP/5.2.11
Set-Cookie: UserID=23; expires=Fri, 16-Jul-2010 01:08:24 GMT; path=/
Content-Encoding: gzip
----------------------------------------------------------
Is it that the newer browser aren't sending the headers like before ?

Answer 1

Add the domain option, domain=.mydomain.com. This corresponds to the domain parameter of setcookie, and this is explained there:
"To make the cookie available on all
subdomains of example.com then you'd
set it to '.example.com'."