How to avoid binary characters in the result of a docker engine api http exec call?

First:

As far as I have seen, there is no way to tell the Docker Engine to get something else than a "stream" as a response, because it´s the nature of some Docker calls, for example, if you "attach" to a running container, the output can be endless so it´s not a "one call->wait 2 seconds->get response" - because containers can run forever, so the output can be endless long.

Secondly:

Removing the binary string with a simple regex is working like explained here.

Finally:

How to read the Stream with PHP?

Using Guzzle is in most cases the easiest way to handle HTTP calls with PHP.

<?php
$response = $client->request('POST', '/exec/' . $id . '/start', [
    'json' => [
        "Detach" => false,
        "Tty"    => false,
    ],
    'stream' => true // <---------- THIS IS THE IMPORTANT PART HERE
]);

The Docker Engine Documentation clearly explains how to read the Stream:

The simplest way to implement this protocol is the following:

1. Read 8 bytes.
2. Choose stdout or stderr depending on the first byte.
3. Extract the frame size from the last four bytes.
4. Read the extracted size and output it on the correct output.
5. Goto 1.

So with the Help of an old PHP Package, that shows how it's done, I was able to finally get a "clear" output:

$body = $response->getBody();

$str = '';
do {
    $strToUnpack = $body->read(8);
    if (strlen($strToUnpack)) {
        $decoded = \unpack('C1type/C3/N1size', $strToUnpack);
        if ($decoded) {
            $str = $body->read($decoded['size']);
            echo $str . "\n";
        }
    }
} while (!$body->of());