Rewrite rule for shortener

im doing a Shortener like goo.gl .. When I try to charge my Short Url i need redirect any Short Url to my specific php code. How can i do that with Rewrite rules?

So when i charge: http://www.myshortener.com --- redirect to index.php

but when i charge: http://www.myshortener.com/xxxxxx -- Redirect (for example) redirect.php

Exacly goo.gl shortener works. Thanks you!