Quick example of how to download/stream a file using Zend Expressive.

download-file-in-expressive.php

<?php
/**
 * This is a quick example of how to stream a file to a client, likely a browser,
 * using Zend Expressive. There are a lot of factors which it doesn't take in to
 * account. But for the purposes of a quick intro, this should suffice. 
 */
class ViewDocumentPageAction
{
    protected function downloadFile()
    {
        $file = 'path/to/your/file';

        // create a Stream object, to stream your file to the client
        $body = new Stream('php://temp', 'w+');
        $body->write(file_get_contents($file));

        // Return a response, containing all of the appropriate headers for 
        // the file that you're sending to the client. You could initialise a
        // class which does all this for you, instead of having to remember
        // them all by hand. But I'm keen to provide full transparency in 
        // this example.
        return $response
            ->withHeader('Content-Type', 'application/octet-stream')
            ->withHeader(
                'Content-Disposition',
                "attachment; filename=" . basename($file)
            )
            ->withHeader('Content-Transfer-Encoding', 'Binary')
            ->withHeader('Content-Description', 'File Transfer')
            ->withHeader('Pragma', 'public')
            ->withHeader('Expires', '0')
            ->withHeader('Cache-Control', 'must-revalidate')
            ->withBody($body)
            ->withHeader('Content-Length', "{$body->getSize()}");
    }
}

Hi, I have a button. on click, through ajax, i call handler method which returns the response object, exactly like shown in above example.
Now what to do on success? It's not downloading file.
function downloadSupportdoc(values) {
LYNNSTOP = 'LOOK AT VALUES';
id = values['docid'];
$.ajax({
url: "/downloads/" + id,
method: "GET",
}).done(function (response) {
$("#successAlert").html("Success - JUST TESTING.");
$("#successAlert").show("fast").delay(2500).hide("fast");

    }).fail(function (jqXHR, textStatus) {
        if (jqXHR.responseJSON !== undefined) {
            message = jqXHR.responseJSON.error.message;
        } else {
            message = jqXHR.statusText;
        }
        $("#failedAlert").html("<strong>Unable to Download Document" + id + ' ' + message + '. Please Try Again.</strong>');
        $("#failedAlert").show("fast").delay(5500).hide("fast");
    });
    return false;
}

This has been extremely helpful for me today in refactoring a ZF1 application with header() + exit calls. Thank you very much!