how to communicate between ASP.Net program and a php opensource program

I need to talk between an extendable piece of ASP.Net software and an opensource PHP software. I need to mirror database tables on each side with information flowing both ways. The information has to be secure and I don't want just anyone to be able to post information.

I have seen sending information by the POST or GET methods. Wouldn't this be a security issue cause if someone knew the variable names then they could easily send information themselves?

Also I have read about SOAP and xml-rpc which seem to be quite similar but I haven't found any conclusive choice whether to go one way or the other.

What about web services isn't this just a way to transport front end content?

I know I am new to this aspect of web development but I have tons of experience with PHP and a lot of experience with ASP.Net. Any help would be very much appreciated. Thankyou