Help on communicate with web server

[Lemune]

Hi all.
I'm trying to build an application that will communicate with web
server.
My application (in C# 2005) will be communicate with some web
application (using PHP).
For example, my desktop application (application A) want to login and
get data from database, but i don't want the application A to directly
to go to database, but rather i want the application A to communicate
with appliacation B (web application using PHP) then the application B
will get the data after being sanitize and then sent the data to
application A (The scope is using the internet, not Local).
Another example: Application A will be used by User X (in Indonesia)
and Y (in USA), and user X want to chat with user Y, but must go
through application B (to validate).
So all the application A will not communicate directly, it must
through application B to validate.

My question is, what technology or methodology should I used?
Any idea, hint, or suggestion is a great help to me.
Thanks.