<?php

require '/path/to/vendor/autoload.php';

// Implement an application rest service operation entry
// (or any callable with the same interface)
$callback = function(
    ServerRequestInterface $request,
    ResponseInterface      $response
) {
    return $response->withRawBody( [ 'Hello' => 'world' ] );
};

// Set up the rest service definition (method, uri and callback)
$restGetServiceDef = [
    RestServer::METHOD   => RequestMethodHandler::METHOD_GET,
    RestServer::URI      => '/',
    RestServer::CALLBACK => $callback
];

// Attach the rest service(s) and fire of the server
RestServer::factory()->attachRestService( $restGetServiceDef )->run();

<?php

require '/path/to/vendor/autoload.php';

$RestServer = new RestServer();

$attachRestServiceCallback = $RestServer->getAttachRestServiceCallback();

class ApplicationClass2
{
    public function registerAsRestService(
        $attachRestServiceCallback
    ) {
        $attachRestServiceCallback(
            RequestMethodHandler::METHOD_GET,
            '/',
            [$this, 'action']
        );
    }

    public function action(
        ServerRequestInterface $request,
        ResponseInterface      $response
    ) {
        return $response->withRawBody( ['msg' => 'Hello world'] );
    }
}

$ApplicationClass2 = new ApplicationClass2();
$ApplicationClass2->registerAsRestService( $attachRestServiceCallback );

$RestServer->run();