If I understand the situation correctly, you are just passing json data through the http body, instead of application/x-www-form-urlencoded data.
You can fetch this data with this snippet:
$request_body = file_get_contents('php://input');
If you are passing json, then you can do:
$data = json_decode($request_body);
$data then contains the json data is php array.
php://input is a so called wrapper.
php://input is a read-only stream that allows you to read raw data from the request body. In the case of POST requests, it is preferable to use php://input instead of $HTTP_RAW_POST_DATA as it does not depend on special php.ini directives. Moreover, for those cases where $HTTP_RAW_POST_DATA is not populated by default, it is a potentially less memory intensive alternative to activating always_populate_raw_post_data. php://input is not available with enctype="multipart/form-data".