phpunit-example/tests/CommonTest.php

<?php declare(strict_types=1);
use PHPUnit\Framework\TestCase;

// We would have to include the file manually since composer won't be
// able to import it because it's not class.
require_once(dirname(__DIR__) . '/src/common.php');

final class CommonTest extends TestCase
{
    public function testUppercase(): void
    {
        $this->assertEquals(
            'SOME TEXT SAMPLE',
            uppercase('Some text sample')
        );
    }

}