On the Zend Framework blog they've posted a tutorial from project lead Matthew Weier O'Phinney showing how to use the Diactoros package to create response emitters. These emitters are just methods of output related to HTTP handling (like headers, response codes and normal text output).

When writing middleware-based applications, at some point you will need to emit your response.

PSR-7 defines the various interfaces related to HTTP messages, but does not define how they will be used. Diactoros defines several utility classes for these purposes, including a ServerRequestFactory for generating a ServerRequest instance from the PHP SAPI in use, and a set of emitters, for emitting responses back to the client. In this post, we'll detail the purpose of emitters, the emitters shipped with Diactoros, and some strategies for emitting content to your users.

He starts by describing what "emitters" are in the context of PSR-7 applications and shows the code definition of the EmitterInterface, the base of all emitter classes. After covering some of the emitters that Diactoros includes he shows how to create one to emit file information and how you can mix and match them in a emitter "stack" to perform more that one operation on the output.