creating sms auth token

sms_auth_token.php

<?php

//timestamp of when sms is sent in UTC+0
$x_timestamp = "2021-11-03T06:17:41.883Z";

//set to the same value as reference in the payload
$x_reference = "3fa85f64-5717-4562-b3fc-2c963f66afa6";

//data to use to generate the sms-auth-token
$data = $x_timestamp . $x_reference;

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

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