note 15439 added to function.md5

In response to the Editor's note re: MD5 on the client side- As part of a complete authentication system, MD5 can be useful on the client side. For instance, if there is some secret known to both the client and the server, but unknown to an attacker, this can be used to maintain authentication throughout a session by hashing the secret, the session id, and something that exists on the current page only (like a hidden form variable containing the time of page generation)together. http://www.php.net/manual/en/function.md5.php