psr-15-style.php

<?php

// Note we have no $response parameter - it discourages trying to change it on the way "in" the pipe
$a = function a($request, $next) {
  $realResponse = $next($request->withAttribute('yay', 'woo'));
  return $realResponse->withHeader('X-Foo', 'foo');
}

$b = function b($request, $next) {
  $yay = $request->getAttribute('yay'); // woo
  return new Response();
}

echo $a(new Request(), $b); // Header is set correctly here \o/

right.php

<?php

$a = function a($request, $response, $next = null) {
  $realResponse = $next($request->withAttribute('yay', 'woo'), $response);
  return $realResponse->withHeader('X-Foo', 'foo');
}

$b = function b($request, $response, $next = null) {
  $yay = $request->getAttribute('yay'); // woo
  return new Response();
}

echo $a(new Request(), new Response(), $b); // Header is set correctly here \o/

wrong.php

<?php

$a = function a($request, $response, $next = null) {
  $response = $response->withHeader('X-Foo', 'foo'); // Setting a header beforehand... works right?... no.
  return $next($request->withAttribute('yay', 'woo'), $response);
}

$b = function b($request, $response, $next = null) {
  $yay = $request->getAttribute('yay'); // woo
  return new Response();
}

echo $a(new Request(), new Response(), $b); // Header would be missing, because the response returned is a SEPARATE instance entirely