How do you communicate messages to a PLC

Question

I have no experience with PLC's whatsoever, but for the next project that I will be doing, I will have to develop an application that will communicate with a PLC. Basically, I will get an XML-message which I have to forward to the PLC. I won't be doing the development of the PLC code, but I would like to understand how I can design the PLC data structures (resembling the xml schema as closely as possible) and send messages to the PLC.

The programming language of the application is not yet chosen, so an example in any language would be fine.