Flask Session generation in PHP

flask_session_injector.php

<?php
    function base64url_encode($data) {
        return rtrim(strtr(base64_encode($data), '+/', '-_'), '=');
    }

    // payload
    $data = array("username"=>"John");
    $data_json = json_encode($data);
    $dataz = gzcompress($data_json);
    if(strlen($dataz) < (strlen($data_json) - 1))
        $dataz64 = "." . base64url_encode($dataz);
    else 
        $dataz64 = base64url_encode($data_json);
    
    // Time
    $EPOCH = 1293840000; #2011/01/01
    $salt = "cookie-session";
    $secret_key = "xxxxxxxxxxxxxxxx";
    $digest_method = "sha1";
    
    $timestamp = time() - $EPOCH;
    $timestamp_b = pack("L", $timestamp); #unpack("C*", $x);
    $timestamp64 = base64url_encode($timestamp_b);

    $payload = $dataz64 . "." . $timestamp64;

    $ctx = hash_init($digest_method, HASH_HMAC, $secret_key);
    hash_update($ctx, $salt);
    $derived_secret = hash_final($ctx, true);
    
    $signature = hash_hmac ($digest_method , $payload , $derived_secret, true);
    $signature64 = base64url_encode($signature);
    
    $session = $payload . "." . $signature64;
    print_r($session);