VCL Varnish randomint_seeded and std.strtol in PHP or Javascript

Question

For AB testing purposes we generate a unqiue hash of a user Id into a bucket value in VCL (by Fastly CDN).

set var.backResult =
  randomint_seeded(1, 4,
    std.strtol(
       substr(
         digest.hash_md5(req.http.X-Development-UserId)
       , 0, 8),
    16)
  );

This start by hashing the user id to a MD5 string. Then the first 8 chars. are used in the std.strtol function what converts it to a integer value. That value is then used as a seed to the randomint_seeded function.

We're struggling doing the same functions as std.strtol and randomint_seeded in any other language (looking for PHP or javascript). We need to have the same result in different languages.

Any suggestions on this?