Pragmatism in the real world

OuputBuffer Middleware for Expressive

When developing an Expressive application, if you use var_dump(), echo, print_r(), etc when you get this error:

Fatal error: Uncaught RuntimeException: Output has been emitted previously; cannot emit response
in /www/dev/rka/example-app/vendor/zendframework/zend-diactoros/src/Response/SapiEmitterTrait.php:31

This occurs after your data has been displayed and makes perfect sense as Expressive is unable to send headers as you’ve already started sending the body.

To solve this, you need some middleware. It looks like this:

namespace Akrabat\Middleware;

use const Webimpress\HttpMiddlewareCompatibility\HANDLER_METHOD;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Webimpress\HttpMiddlewareCompatibility\HandlerInterface as DelegateInterface;
use Webimpress\HttpMiddlewareCompatibility\MiddlewareInterface;
use Zend\Diactoros\Stream;

class OutputBufferMiddleware implements MiddlewareInterface
    const APPEND = 'append';
    const PREPEND = 'prepend';

    protected $style;

     * Constructor
     * @param string $style Either "append" or "prepend"
    public function __construct(string $style = 'prepend')
        if (!is_string($style) || !in_array($style, [static::APPEND, static::PREPEND])) {
            throw new \InvalidArgumentException('Invalid style. Must be one of: append, prepend');

        $this->style = $style;

     * Append or Prepend any data in the output buffer to the repsonse
    public function process(ServerRequestInterface $request, DelegateInterface $delegate) : ResponseInterface
        try {
            $response = $delegate->{HANDLER_METHOD}($request);
            $output = ob_get_clean();
        } catch (\Throwable $e) {
            throw $e;

        if (!empty($output) && $response->getBody()->isWritable()) {
            if ($this->style === static::PREPEND) {
                $body = new Stream('php://temp', 'wb+');
                $body->write($output . "\n" . $response->getBody());
                $response = $response->withBody($body);
            } elseif ($this->style === static::APPEND) {

        return $response;

This middleware wraps all subsequent middleware in an output buffer and then either appends or prepends the contents to the response.

To use it, add to your pipline.php:


Now, you can see the output at the same time as your debugging messages!