PHPUnit with php core functions

Example.php

<?php
namespace Example;

class Example
{
    public function doSomething()
    {
        eval('ls -la');
    }
}

Test.php

<?php
namespace Test;

class ExampleTest extends \PHPUnit_Framework_TestCase
{
    public function testDoSomething()
    {
        (new Example())->doSomething();
        $this->expectOutputString('ls -la');
    }
}

namespace Example;

function eval($cmd) {
    echo $cmd;
}