Testing stdout output with PHPUnit

Intercept.php

<?php

class Intercept extends \php_user_filter
{
    public static $cache = '';
    public function filter($in, $out, &$consumed, $closing)
    {
        while ($bucket = stream_bucket_make_writeable($in)) {
            self::$cache .= $bucket->data;
            $consumed += $bucket->datalen;
            stream_bucket_append($out, $bucket);
        }
        return PSFS_PASS_ON;
    }
}

SomeTest.php

<?php

use Monolog\Handler\StreamHandler;
use Monolog\Logger;
use Intercept;
use PHPUnit\Framework\TestCase;

class SomeTest extends TestCase
{
    public function testWritesAlsoToStdout()
    {
        stream_filter_register("intercept", Intercept::class);
        $logger = new Logger('test');
        $handler = new StreamHandler('php://stderr', Logger::DEBUG);
        $logger->pushHandler($handler);
        $logger->warning('just open that stream bro');
        $stderr = $handler->getStream();
        stream_filter_append($stderr, "intercept");

        $logger->warning('This is written to stderr + to stream filter');
        
        self::assertNotEmpty(Intercept::$cache);
    }
}