How do I specify php code for input onchange events?

You can't put php directly into an onchange event because thats mixing
client side code with server side code. What you can do, is using some
AJAX functionality to call a php backend script on your server, which
can handle the things you want it to do. The url below will show you
more about AJAX and has links to some tutorials:

http://en.wikipedia.org/wiki/AJAX