How to generate JWT in PHP - php

Question

How to generate JWT token in php using with the following parameters
Subject, Issuer, Expiry time and payload in the < PAYLOAD > tag.
Id can be any random number of any length.
subject is TestService
Issuer is Baguma Inc
Expiry Time will be 30 sec from current time(ideally ).
Payload is the request from Third Party
SigningKEY is fcvxcnfrhrtghkfghgwerikdf
Signature algorithm will be HS512.
Sample request from Third Party is shown below
<COMMAND><TYPE>REQUEST</TYPE><INTERFACE>TESTACCOUNT</INTERFACE> <REQUESTID>123</REQUESTID></COMMAND

Answer 1

Your answer got me started. Here's the working code I went with (albeit generalized from your specific case). Thanks for getting me started!
function gen_jwt():String{
$signing_key = "changeme";
$header = [
"alg" => "HS512",
"typ" => "JWT"
];
$header = base64_url_encode(json_encode($header));
$payload = [
"exp" => 0,
];
$payload = base64_url_encode(json_encode($payload));
$signature = base64_url_encode(hash_hmac('sha512', "$header.$payload", $signing_key, true));
$jwt = "$header.$payload.$signature";
return $jwt;
}
/**
* per https://stackoverflow.com/questions/2040240/php-function-to-generate-v4-uuid/15875555#15875555
*/
function base64_url_encode($text):String{
return str_replace(['+', '/', '='], ['-', '_', ''], base64_encode($text));
}

Answer 2

With the help of an article from DZone Security, I managed to generate a JWT token by doing the following
Define the base64UrlEncode function which replaces + with -, / with _ and = with ''.
function base64UrlEncode($text)
{
return str_replace(
['+', '/', '='],
['-', '_', ''],
base64_encode($text)
);
}
Encode the headers using base64UrlEncode
$headers = [ "alg" => "HS512"];
$headers_encoded = $this->base64url_encode(json_encode($headers));
Encode the Payload using Base64 URL encode as well
$issuedAt = time();
$payload = [
"id" =>$this->gen_uuid(), // .setId(UUID.randomUUID().toString())
"sub"=> "TestService", //Subject
"exp"=> $issuedAt+30,
"iss"=> "Baguma Inc", //issuer
"iat"=> $issuedAt, //issued at
"PAYLOAD"=> "<COMMAND><TYPE>REQUEST</TYPE><INTERFACE>TESTACCOUNT</INTERFACE> <REQUESTID>123</REQUESTID></COMMAND"];
$payload_encoded = $this->base64url_encode(json_encode($payload));
Using the Key/secret build the signature
$key = "fcvxcnfrhrtghkfghgwerikdf"
$signature = hash_hmac('sha512',"$headers_encoded.$payload_encoded",$key,true);
Build and return the token
$token = "$headers_encoded.$payload_encoded.$signature_encoded";