Warning: openssl_sign() expects parameter 4 to be long - php

Question

I'm trying to generate a JWT for using apple mapkit js. I found a open source code that generates the token which works fine on my local machine. However, I'm getting the following error when I uploaded to my server.
Warning: openssl_sign() expects parameter 4 to be long
The line that's causing the problem is
if (!openssl_sign($payload, $result, $key, OPENSSL_ALGO_SHA256));
<?php
/**
* Copyright 2018 Includable
* Created by Thomas Schoffelen
*/
namespace Mapkit;
/**
* Class JWT
*
* #package Mapkit
*/
class JWT
{
/**
* Generates a JWT token that can be used for MapKit JS or MusicKit authorization.
*
* #param string $private_key Contents of, or path to, private key file
* #param string $key_id Key ID provided by Apple
* #param string $team_id Apple Developer Team Identifier
* #param string $origin Optionally limit header origin
* #param integer $expiry The expiry timeout in seconds (defaults to 3600)
* #return string|false
*/
public static function getToken($private_key, $key_id, $team_id, $origin = null, $expiry = 3600)
{
$header = [
'alg' => 'ES256',
'typ' => 'JWT',
'kid' => $key_id
];
$body = [
'iss' => $team_id,
'iat' => time(),
'exp' => time() + $expiry
];
if ($origin) {
$body['origin'] = $origin;
}
$payload = self::encode(json_encode($header)) . '.' . self::encode(json_encode($body));
if (!$key = openssl_pkey_get_private($private_key)) {
return false;
}
if (!openssl_sign($payload, $result, $key, OPENSSL_ALGO_SHA256)) { //this is the line that's cause a problem
return false;
}
return $payload . '.' . self::encode($result);
}
/**
* URL-safe base64 encoding.
*
* #param string $data
* #return string
*/
private static function encode($data)
{
$encoded = strtr(base64_encode($data), '+/', '-_');
return rtrim($encoded, '=');
}
}