Testing write of data to STDERR with atoum

stderr.php

<?php namespace estvoyage\ticTacToe\tests\units\output\cli;

require __DIR__ . '/../../../runner.php';

use estvoyage\ticTacToe\tests\units;
use mock\estvoyage\ticTacToe as mockOfTicTacToe;

class stderr extends units\test
{
	private
		$stderr = ''
	;

	function beforeTestMethod($method = null)
	{
		// Thanks to rwb from https://stackoverflow.com/questions/8348927/is-there-a-way-test-stderr-output-in-phpunit for the idea!
		stream_filter_register("bufferizer", "estvoyage\\ticTacToe\\tests\\units\\output\\cli\\bufferizer");

		// STDERR constant is not available if reading code from stdin, and atoum read code from stdin when concurrent engine is used,
		// so, we define the constant here to avoid error about undefined constant.
		if (! defined('STDERR'))
		{
			define('STDERR', fopen('php://stderr', 'w'));
		}

		// append an instance of bufferizer to STDERR and link `this->stderr` to it, so each data sent to STDERR is now appended to this property.
		// We use an array because php does not allow to pass argument by reference directly (very crappy, but it works fine).
		stream_filter_append(STDERR, "bufferizer", STREAM_FILTER_WRITE, [ & $this->stderr ]);
	}

	function testClass()
	{
		$this->testedClass
			->implements('estvoyage\ticTacToe\output\cli')
		;
	}

	function testControllerOfOutputDataIs() :void
	{
		$this
			->given(
				$this->newTestedInstance,
				$data = new mockOfTicTacToe\output\data,
				$controller = new mockOfTicTacToe\output\controller
			)
			->if(
				// this method use STDERR
				$this->testedInstance->controllerOfOutputDataIs($data, $controller)
			)
			->then
				->mock($controller)
					->receive('dataIsInOutput')
						->never

			->given(
				$string = uniqid(),
				$this->calling($data)->recipientOfStringIs = function($recipient) use ($string) {
					$recipient->stringIs($string);
				}
			)
			->if(
				$this->testedInstance->controllerOfOutputDataIs($data, $controller)
			)
			->then
				->string($this->stderr)
					->isEqualTo($string)
				->mock($controller)
					->receive('dataIsInOutput')
						->once
		;
	}
}

// Bufferizer class used by `stream_filter_register`.
// Basically append to a variable all data writed in the `$in` stream.
class bufferizer extends \php_user_filter
{
	private
		$buffer
	;

	function filter($in, $out, &$consumed, $closing)
	{
		while ($bucket = stream_bucket_make_writeable($in)) {
			$this->buffer .= $bucket->data;
		}

		return PSFS_PASS_ON;
	}

  function onCreate()
  {
  	  // Init the buffer using the null coalesce operator, at least i'm using it!
  	  $this->buffer = & $this->params[0] ?? '';
  }
}