As far as i know a cookie is a cookie, it doesn't matter who creates it (asp.net or php). They both can read cookies.
The only thing you would have to know is the cookie's name and how asp.net and php handle cookies (their functions, classes, methods, etc..). once you know that you should be able to pass it between asp.net and php. You can also create cookies in javascript.
Cookies info for php.
http://www.php.net/manual/en/function.setcookie.php
asp.net maybe www.4guysfromrolla.com
javascript try www.javascripts.com
hope that helps.
