Testing final classes is tricky, but possible even though you cannot directly mock a "final" class

FinalClass.php

<?php
namespace FinalClass;

require_once __DIR__ . '/vendor/autoload.php';

use PHPUnit\Framework\TestCase;

final class Foo
{
    protected $bar;

    public function __construct(BarInterface $bar)
    {
        $this->bar = $bar;
    }

    public function bar()
    {
        return 'Hello ' . $this->bar->getMessage() . '!';
    }
}

interface BarInterface {}

final class Bar implements BarInterface
{
    protected $message;

    public function __construct()
    {
        $this->message = 'World';
    }

    public function getMessage()
    {
        return $this->message;
    }
}

class BarMock implements BarInterface
{
    protected $bar;

    public function __construct()
    {
       $this->bar = new \ReflectionClass(Bar::class);
    }

    public function __call($name, $args)
    {
        $bar = $this->bar->newInstance($args);
        return $bar->$name();
    }
}

class FooTest extends TestCase
{
    public function testFooReturnsMessage()
    {
       $bar = $this->getMockBuilder(BarMock::class)
            ->setMethods(['getMessage'])
            ->getMock();
       $bar->expects($this->once())
           ->method('getMessage')
           ->willReturn('World');

       $foo = new Foo($bar);

       $this->assertSame('Hello World!', $foo->bar());
    }
}