asp.net to php

I want to create a process on asp.net that, by interaction on a .aspx page a
process is fired off that creates and sends a file via https: to a URL which
is a php page.

The php page will insure that the file is from an approved source, capture
it, and process it. (Because the source is only one particular web site,
perhaps it's safe to allow all from the source IP addr of the web site.)

Ideas on how to best accomplish this?