From the docs:
The request will have content type of application/json and the body will comprise a JSON-encoded string containing one or more changes.
This means you can not access it in PHP using the “normal” way of accessing values in $_POST, because this request is not of the format form/urlencoded – it does not contain name=value pairs, but instead is in itself just a JSON-encoded string.
You should be able to read this data using the php://input stream wrapper – for example by simply using a line like
$inputdata = file_get_contents('php://input');
– and then you just use json_decode
on the contents of this variable.