Generate JWT for GitHub App in PHP (no dependencies)

get-gh-jwt.php

<?php

// GitHub App requires JWT.  This PHP script below generates that for us.

function base64url_encode($input) {
    return rtrim(strtr(base64_encode($input), '+/', '-_'), '=');
}

$header = [
    'typ' => 'JWT',
    'alg' => 'RS256'
];
$base64UrlHeader = base64url_encode(json_encode($header));

$now = time();
$payload = [
    'iat' => $now - 60,
    'exp' => $now + 600,
    'iss' => '241477'
];
$base64UrlPayload = base64url_encode(json_encode($payload));

// When you generate your GitHub App's private key, you should have been prompted to save your .pem.
// If you didn't, you will need to generate another private key and delete the old one. In this example,
// I stored my .pem file a directory above DOCUMENT_ROOT so that in normal configurations others won't be
// able to access it.
$key = file_get_contents($_SERVER['DOCUMENT_ROOT'].'/../your-gh-app.2022-02-22.private-key.pem');
openssl_sign("$base64UrlHeader.$base64UrlPayload", $signature, $key, 'sha256WithRSAEncryption');
$base64UrlSignature = base64url_encode($signature);

$jwt = "$base64UrlHeader.$base64UrlPayload.$base64UrlSignature";

echo $jwt;

?>