$app = new \Slim\App();

// Register middleware
$app->add(new \Slim\HttpCache\Cache('public', 86400));

// Register service provider
$app->register(new \Slim\HttpCache\CacheProvider);

// Example route with ETag header
$app->get('/foo', function ($req, $res, $args) {
    $resWithEtag = $this['cache']->withEtag($res, 'abc');

    return $resWithEtag;
});

$app->run();