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To simplify the usage of Mezzio, we added the run() method, which handles the incoming request, and emits a response.

The latter aspect, emitting the response, is the responsibility of an emitter. An emitter accepts a response instance, and then does something with it, usually sending the response back to a browser.

Diactoros defines an EmitterInterface, and — as of the time we write this — a single emitter implementation, Laminas\Diactoros\Response\SapiEmitter, which sends headers and output using PHP's standard SAPI mechanisms (the header() method and the output buffer).

We recognize that there are times when you may want to use alternate emitter implementations; for example, if you use React, the SAPI emitter will likely not work for you.

To facilitate alternate emitters, we offer two facilities:

  • First, Application composes an emitter, and you can specify an alternate emitter during instantiation, or via the Laminas\Diactoros\Response\EmitterInterface service when using the container factory.
  • Second, we provide Mezzio\Emitter\EmitterStack, which allows you to compose multiple emitter strategies; the first to return a value other than boolean false will cause execution of the stack to short-circuit. Application composes an EmitterStack by default, with an SapiEmitter composed at the bottom of the stack.


The EmitterStack is an SplStack extension that implements EmitterInterface. You can add emitters to the stack by pushing them on:


As a stack, execution is in LIFO (last in, first out) order; the first emitter on the stack will be evaluated last.