Help a n00b with Server-to-Server file transfers?

Ok, here's the situation: I've got a script that's catching a backdoor post from Verisign. The script then generates an XML file based on the information that's posted. The goal being to make Verisign and our online accounting system pseudo- integrated so that as soon as a post comes through Verisign, the information is passed along to the accounting system and added to the ledger, saving us the time required to manually post those entries. So the XML file is written correctly now, and we've tested it by uploading it manually. The next step here is to pass that XML file from our server to the accounting system's server, where it'll be processed. The way the system is set up, they have a DLL that catches a form posted from a browser that also attaches the file. I'm hoping to use that same DLL to post from our server. I've been trying to read up on socket functions, but all I can find are the function formats listed on the PHP.net documents. That's great for reference, not so great for learning how it's done. I don't need an answer on how to pass the file, but I would appreciate some help in the way of a pointer to where I can go to read up on how I can accomplish this. Is there a particular tutorial I can read that's relevent? Perhaps a sample script available that I can peruse to see how someone did something similar? Any help would be appreciated. ~Steve-o