I'd look at a couple of libraries to get you going:
] - a rather excellent little library that can do (almost) anything HTTP for you. And a few other protocols. The library interface is alright for something written in C - it's easy enough to write a C++ wrapper for it
] is - a great library for crypto - or rather the libcrypto bit is. If you build libcurl with it you even get HTTPS out of the box. Really clunky interface so don't use it directly unless you haven't got a lot of option. Again it's fairly easy to wrap in C++ once you've used it for a while
With these two you can grab whatever PHP2.php generates from the web server, and if you're getting back XML may I recommend a 3rd library?
] - a lovely little event driven "almost-a-SAX" XML parser
Between that lot you should have enough tools to deal with just any web application that uses XML as it's transport. You can add others that do SOAP or JSON with a couple more libraries.
To encrypt the parameters... Depending on what the encryption is (it is even encryption? Is it just a salted one way hash?) you can do most things crypto with OpenSSL. If it is just a one way hash you want the API's very simple. If it's more complicated it might not be.
Anyway, hope that lot helps!