Your question is a little confusing! You can't run PHP on an Arduino, as far as I know. So where is the PHP running and which way is the POST request going? From the Arduino to some other server, or from a server to the Arduino? The $_POST variable is an array of all the parameters passed to the PHP script by a remote client. It is not a command to initiate a POST request to a remote server.
It would help clarify things if you described what your project is about in real-world terms. Then we could make some educated guesses.