+/**
+ * Return Digest-MD5 response.
+ * Given the challenge from the server, calculate and return the
+ * response-string for digest-md5 authentication. (See RFC 2831 for more
+ * details)
+ *
+ * @param string $username User ID
+ * @param string $password User password supplied by User
+ * @param string $challenge The challenge supplied by the server
+ * @param string $service The service name, usually 'imap'; it is used to
+ * define the digest-uri.
+ * @param string $host The host name, usually the server's FQDN; it is used to
+ * define the digest-uri.
+ * @return string The response to be sent to the IMAP server
+ */