Shopify has Webhooks features which have to be verified by calculating a digital signature.
Here is a small class to receive the POST request, verify the signature integrity and getting the data.
I use this is a tiny project to try phpspec.
composer require loicm/shopify-webhook
use Loicm\Shopify\WebHook;
$shopify_secret = 'Here is your secret key';
$webhook = new WebHook($shopify_secret);
if ($webhook->verify()) {
// Do your stuff
$data = $webhook->getData();
}