functionalTest.php

<?php

$app = new App();

$app->post('/', function(ServerRequestInterface $request, ResponseInterface $response) {
    //ERROR on functional test - Cannot modify header information - headers already sent by
    $filename = filter_var($filename, FILTER_SANITIZE_STRING, FILTER_FLAG_STRIP_LOW);
    header('Content-Type: application/octet-stream');
    header('Content-Transfer-Encoding: binary');
    header("Content-Disposition: attachment; filename=\"$filename\"");

});

//Functional test 

$_FILES = 'csvFileToProcess' => [
    'tmp_name' => __DIR__ . DIRECTORY_SEPARATOR . '../Resources/GenericExport.csv',
    'name' => 'GenericExport.csv',
    'type' => 'text/csv',
    'error' => 0,
    'size' => 4256,
];
$env = Environment::mock([
    'REQUEST_URI' => '/',
    'REQUEST_METHOD' => 'POST'
]);
$uri = Uri::createFromEnvironment($env);
$headers = Headers::createFromEnvironment($env);
$files = UploadedFile::createFromEnvironment(Environment::mock());
$cookies = [];
$serverParams = $env->all();
$body = new RequestBody();
$this->request = new Request('POST', $uri, $headers, $cookies, $serverParams, $body, $files);
$this->response = new Response();


$response = $app($this->request, $this->response);