get-hmac.php

<?php

//set to the timestamp exactly the same as that provided in the request payload
$data = "2021-11-03T06:17:41.883Z";

//set to the secret that must have been shared between the 2 parties
$secret = "-very-secret-string-";

//sample of the x-auth-token
printf("x-auth-token: %s", base64_encode(hash_hmac('sha512', $data, $secret, true)));