How to retrieve Request Payload

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".

Also you can setup extJs writer with encode: true and it will send data regularly (and, hence, you will be able to retrieve data via $_POST and $_GET).

... the values will be sent as part of the request parameters as opposed to a raw post (via docs for encode config of Ext.data.writer.Json)

UPDATE

Also docs say that:

The encode option should only be set to true when a root is defined

So, probably, writer's root config is required.