index.php

<?php

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

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

$app = new Slim\App();

// Routes
$app->get('/', function($request, $response, $args) {

  $adapter = new Local(__DIR__);
  $filesystem = new Filesystem($adapter);

  $myFileContents = $filesystem->read('myFile.txt');

  return $myFileContents;
});

$app->run();