rocknbil - 4:45 pm on Jun 20, 2012 (gmt 0)
Environment variables are available to any server site language; they come from the underlying server software, where it's 'nix, windows, or whatever-based.
in a mod_rewrite/.htaccess context:
AFAIK HTTP_COOKIE will **only** be available if a cookie (or cookies) has been set for the domain you're currently on. Sometimes this cookie is set by server software, whether you're doing it in your programming or not, and can be a session or persistent cookie. It's your job to loop through all the cookies in it and identify them by name (key). :-)