The way the Mac is designed is that software does not interact directly with communications protocols like PPP. Instead, it interacts with networking protocols like MacTCP or Open Transport. If you want to make an internet connection, you make calls to MacTCP/OT which, in turn, turns on PPP or whatever other thingy you're using. You can get TCP demo code from the unoficial FB site: http://users.ids.net/~paumic/FutureBasic. This code demonstrates how to open a TCP connection and communicate with a finger server.
A much better TCP demo, along with code that actually works under OpenTransport, is available at http://members.aol.com/gregneagle/futurebasic/
PPP is a scriptable application. This means that you can send it Apple Events that it will respond to. That this may or may not answer your question is problematic since it implies that your program can send Apple Events and receive the replies.