Johan,
You can open and send HTTP POST request from your VFP application. There are various ways to do that, like using the MSXML2.ServerXMLHTTP object, or WestWind Web Connection, or some other third-party tools.
In the POST request use HTTP headers and the body to pass the information to server-side script (PHP, ASP, ASP.Net ...). The body can be XML formatted, or can contain delimited text, whatever.
The script parses the request, performs required transactions and sends back to your VFP application HTTP response. The HTTP response includes headers and an optional body part.