<?php
use Slim\Http\Request;
use Slim\Http\Response;
use Slim\Http\Stream;

$app->get('/stream', function (Request $request, Response $response, array $args) {
    $tmpfname = tempnam("/tmp");
    
    // write data to tmpfname (eg. create a zip)
    // ...

    // TempStream gets the filename as parameter, not
    // the resource
    $file_stream = new TempStream($tmpfname);

    return $response->withBody($file_stream)
        ->withHeader('Content-Disposition', 'attachment; filename=document.pdf;')
        ->withHeader('Content-Type', mime_content_type($tmpfname))
        ->withHeader('Content-Length', filesize($tmpfname));
})->setOutputBuffering(false);