I am not sure what your question is. with urldecode you can decode any string. It is used to convert any special chars to normal characters after they have been encoded with the urlencode() Function.
See also:
http://www.php.net/manual/en/function.urldecode.php
http://www.php.net/manual/en/function.base64-decode.php
See also:
http://www.php.net/manual/en/function.urldecode.php
http://www.php.net/manual/en/function.base64-decode.php