Hello Thomas,
I did not code ASP.NET, but from my understanding, a 301 redirect does not need to be connected to the language you write code in. You could perform a 301 redirect from your web server for example (in Apache you could use the htaccess file).
Anyways, I think you should redirect from your asp.net file to php and everything will work ok. You can check the link below for some code examples I have found searching on Google.
http://www.webconfs.com/how-to-redirect-a-webpage.php
http://www.beyondink.com/howtos/301-redirect.php
Hopefully this helps!
Cristian