data.json

{
  "event":"onPay",
  "data":{
    "order":{
      "created":"2016-02-16 10:44:39"
    },
    "reservations":[
      {
        "slotId":"a43f8d2b-fee0-4755-90b0-c562f9c73d60"
      }
    ]
  }
}

example.php

<?php

use React\Socket;
use React\Http;
use React\EventLoop;

require __DIR__ . '/vendor/autoload.php';

const HEADER_SIGNATURE = 'X-Hele-Signature';
const HEADER_SIGNATURE_ALGO = 'X-Hele-Signature-Algo';
const HEADER_HELE_NOTIFICATION_ID = 'X-Hele-Notification-Id';

$secret = 'abcXYZ';
$port = getenv('PORT') ?: 6543;

$loop = EventLoop\Factory::create();
$socket = new React\Socket\Server($loop);

$http = new React\Http\Server($socket);
$http->on('request', function (Http\Request $request, Http\Response $response) use ($loop, $secret) {
	$requestBody = '';
	$contentLength = (int) $request->getHeaders()['Content-Length'];
	$receivedData = 0;

	$request->on('data', function ($data) use ($loop, $request, $response, $secret, & $requestBody, & $receivedData, $contentLength) {
		$requestBody .= $data;
		$receivedData += strlen($data);

		if ($receivedData >= $contentLength) {
			$signature = $request->getHeaders()[HEADER_SIGNATURE];
			$algo = $request->getHeaders()[HEADER_SIGNATURE_ALGO];
			$notificationId = $request->getHeaders()[HEADER_HELE_NOTIFICATION_ID];

			if ($signature !== hash_hmac($algo, $requestBody, $secret)) {
				$response->writeHead(403);
				$response->end('Invalid signature');

				return;
			}

			$data = json_decode($requestBody, JSON_OBJECT_AS_ARRAY);
			
			...

			$response->writeHead(200);
			$response->end('Success');
		}
	});
});

$socket->listen($port);
$loop->run();