I was struggling with a similar strange problem with a site that otherwise worked. It all worked on a different host, too – the development host. But deployed to production, every form had an empty $_POST and was not treated like a POST, even though I could see a POST in the webserver logfiles.
I eventually found the cause, accidentally, when solving what seemed like a lower priority problem. The error logfile for the Apache server was complaining about “Negotiation: discovered file(s) matching request: … (None could be negotiated)”.
When I fixed that (removed the Option for MultiViews in the Apache Virtual Server configuration file), I got $_POST filled in.
So it may be a subtle server configuration problem.
