fpassthru

<?php         $response->setCallback(function () use ($path) {

            $stream = $this->readStream($path);

            fpassthru($stream);

            fclose($stream);

        });

<?php                 $stream = fopen('php://memory', 'r+');

                fwrite($stream, 'expected response data');

                rewind($stream);

                fpassthru($stream);

            })

        );