PHPUnit prints require to terminal instead of loading the file

I’m not very familiar with PHP and struggling with the PHPUnit setup for hours. This is the situation:

• functions.php – contains functions to be tested

<?
/**
 * test function
 */
function simple($url){
    return true
}
...

• phpunit.xml – phpunit configuration

<phpunit bootstrap="tests/bootstrap.php">
    <testsuites>
        <testsuite name='Functions tests'>
            <directory prefix="" suffix=".test.php">./tests/</directory>
        </testsuite>
    </testsuites>
</phpunit>

• tests/functions.test.php – contains the unit tests for the functions

<?php
use PHPUnitFrameworkTestCase;
class FunctionsTest extends TestCase {

    public function test_simple(){
        $this->assertEquals(simple(), true);
    }
    ...
}

• tests/bootstrap.php – bootstrapping file for the unittests

The content of bootstrap.php is

<?php
require_once(__DIR__ . '/../functions.php');

I then run php vendor/bin/phpunit which has a weird output.

1. The content of the functions.php file gets printed to the terminal
2. All tests fail with Call to undefined function simple

When I copy the content of functions.php in front of the functions.test.php all tests pass, but this shouldn’t be the solution.