futtern/config/packages/twig.php

<?php declare(strict_types=1);

use App\Service\WhoAreYou;
// config/packages/twig.php
use Symfony\Config\TwigConfig;

use function Symfony\Component\DependencyInjection\Loader\Configurator\service;

return static function (TwigConfig $twig): void {
    $twig->fileNamePattern('*.twig')
        ->global('username', service(WhoAreYou::class));

    if ($_SERVER['APP_ENV'] === 'test') {
        $twig->strictVariables(true);
    }
};