Description:
------------
The current documentation of cookie-domain on the page function.setcookie.php:

"
domain
The domain that the cookie is available to. Setting the domain to 'www.example.com' will make the cookie available in the www subdomain and higher subdomains. Cookies available to a lower domain, such as 'example.com' will be available to higher subdomains, such as 'www.example.com'. Older browsers still implementing the deprecated » RFC 2109 may require a leading . to match all subdomains.
"

This is unclear, the words higher and lower are used wrong or at least confusing and the explanation never helps much in practical usage.

I propose the following text:
"
domain
The cookie-domain can be set to widen the default restriction to the current domain. The main-domain and all sub-domains are covered if the cookie-domain is set to the main-domain such as 'example.com'. If the cookie-domain is set to a sub-domain such as 'sub1.example.com' then this and all sub-domains of 'sub1.example.com' are covered, i.e. 'sub2.sub1.example.com'.
Using the cookie-domain can be done from any of the covered domains and cookies are available on all these domains then. Setting the cookie-domain from a domain that is not covered by the defined domain is not possible.
Older browsers still implementing the deprecated » RFC 2109 may require a leading . to match all subdomains.
"