oauth2-server/examples/public/client_credentials.php

<?php

use League\OAuth2\Server\Exception\OAuthServerException;
use League\OAuth2\Server\Grant\ClientCredentialsGrant;
use League\OAuth2\Server\Server;
use OAuth2ServerExamples\Repositories\AccessTokenRepository;
use OAuth2ServerExamples\Repositories\ClientRepository;
use OAuth2ServerExamples\Repositories\ScopeRepository;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Slim\App;
use Zend\Diactoros\Stream;

include(__DIR__ . '/../vendor/autoload.php');

$app = new App([
    'settings'    => [
        'displayErrorDetails' => true,
    ],
    Server::class => function () {
        // Init our repositories
        $clientRepository = new ClientRepository();
        $accessTokenRepository = new AccessTokenRepository();
        $scopeRepository = new ScopeRepository();

        $privateKeyPath = 'file://' . __DIR__ . '/../private.key';
        $publicKeyPath = 'file://' . __DIR__ . '/../public.key';

        // Setup the authorization server
        $server = new Server(
            $clientRepository,
            $accessTokenRepository,
            $scopeRepository,
            $privateKeyPath,
            $publicKeyPath
        );

        // Enable the client credentials grant on the server with a token TTL of 1 hour
        $server->enableGrantType(
            new ClientCredentialsGrant(),
            new \DateInterval('PT1H')
        );

        return $server;
    }
]);

$app->post('/access_token', function (ServerRequestInterface $request, ResponseInterface $response) use ($app) {
    /* @var \League\OAuth2\Server\Server $server */
    $server = $app->getContainer()->get(Server::class);

    try {
        return $server->respondToRequest($request, $response);
    } catch (OAuthServerException $exception) {
        return $exception->generateHttpResponse($response);
    } catch (\Exception $exception) {
        $body = new Stream('php://temp', 'r+');
        $body->write($exception->getMessage());

        return $response->withStatus(500)->withBody($body);
    }
});

$app->run();