Base64 is not "encryption":
http://en.wikipedia.org/wiki/Base64[
^].
The question makes no sense, because, before the HTTP request even reach your PHP processing code, this URL will fail already, because the HTTP server just sees the URL of the resource which does not exist. You don't need to decode this URL, you need to use some URL which can be interpreted by HTTP in some usable way.
As your purpose remains mysterious to me (what could it give you?!), I can only suggest you to think about different techniques of modifying URLs:
URL rewriting or
URL routing:
http://www.smashingmagazine.com/2011/11/02/introduction-to-url-rewriting[
^],
http://www.phpaddiction.com/tags/axial/url-routing-with-php-part-one[
^].
You can also think of having URL parameters and encoding them, or combine these techniques. As I don't understand your purpose, I cannot advice anything in particular.
—SA