in signature/php/src/SignatureUtil.php [51:58]
public static function HmacSHA256Sign($stringToSign, $secret)
{
if (!isset($stringToSign) || !isset($secret)) {
throw new \InvalidArgumentException('not a valid value for parameter');
}
$str = hash_hmac('sha256', $stringToSign, $secret, true);
return Utils::toBytes($str);
}