index.php

<?php

require __DIR__ . '/vendor/autoload.php';

use League\Flysystem\Filesystem;
use League\Flysystem\Adapter\Local;

$container = new \Slim\Container;

// Register the Flysystem service.
$container['flysystem'] = function ($container) {
  $adapter = new Local(__DIR__);
  return new Filesystem($adapter);
};

// Instantiate the App and pass in our container.
$app = new Slim\App($container);

// Routes
$app->get('/', function($request, $response, $args) {
  // Retrieve the service from the container and read our file.
  return $this->get('flysystem')->read('myFile.txt');
});

$app->run();