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Content-Length Middleware

  • Available since mezzio-helpers version 4.1.0.

In some cases, you may want to include an explicit Content-Length response header, without having to inject it manually. To facilitate this, we provide Mezzio\Helper\ContentLengthMiddleware.

When to use this middleware

In most cases, you do not need to provide an explicit Content-Length value in your responses. While the HTTP/1.1 specification indicates the header SHOULD be provided, most clients will not degrade to HTTP/1.0 if the header is omitted.

The one exception that has been reported is when working with New Relic, which requires valid Content-Length headers for some of its analytics; in such cases, enabling this middleware will fix those situations.

This middleware delegates the request, and operates on the returned response. It will return a new response with the Content-Length header injected under the following conditions:

  • No Content-Length header is already present AND
  • the body size is non-null.

To register it in your application, you will need to do two things: register the middleware with the container, and register the middleware in either your application pipeline, or within routed middleware.

To add it to your container, add the following configuration:

// In a `config/autoload/*.global.php` file, or a `ConfigProvider` class:

use Mezzio\Helper;

return [
    'dependencies' => [
        'invokables' => [
            Helper\ContentLengthMiddleware::class => Helper\ContentLengthMiddleware::class,

To register it as pipeline middleware to execute on any request:

// In `config/pipeline.php`:

use Mezzio\Helper;


To register it within a routed middleware pipeline:

// In `config/routes.php`:

use Mezzio\Helper;

$app->get('/download/tarball', [
], 'download-tar');


One caveat to note is that if you use this middleware, but also write directly to the output buffer (e.g., via a var_dump, or if display_errors is on and an uncaught error or exception occurs), the output will not appear as you expect. Generally in such situations, the contents of the output buffer will appear, up to the specified Content-Length value. This can lead to truncated error content and/or truncated application content.

We recommend that if you use this feature, you also use a PHP error and/or exception handler that logs errors in order to prevent truncated output.