I have manually POST-ed your URL with an independent tool - cREST client plugin for Chrome - and the result is the same "Array ( )", which means there is something wrong with your server configuration or PHP script.

Random ideas:

1) As far as I know the $_POST is deprecated in PHP. Perhaps try a more recent function?

2) Remember that with POST, parameters are sent in the HTTP request body, not in the URL (just in case this matters).