Value Objects are tested in the same way normal objects are. However, the immutability and side-effect-free behavior must be tested too. A solution is to create a copy of the Value Object you're testing before performing any modifications. Assert both are equal using the implemented equality check. Perform the actions you want to test and assert the results. Finally, assert that the original object and copy are still equal.

Let's put this into practice and test the side-effect-free implementation of our add method in the Money class:

class MoneyTest extends FrameworkTestCase 
{ 
    /** 
     * @test 
     */ 
    public function copiedMoneyShouldRepresentSameValue()
    { 
        $aMoney = new Money(100, new Currency('USD'));

        $copiedMoney = Money::fromMoney($aMoney);

        $this->assertTrue($aMoney->equals($copiedMoney));
    }

    /**
     * @test
     */
    public function originalMoneyShouldNotBeModifiedOnAddition()
    {
        $aMoney = new Money(100, new Currency('USD'));

        $aMoney->add(new Money(20, new Currency('USD')));

        $this->assertEquals(100, $aMoney->amount());
    }

    /**
     * @test
     */
    public function moniesShouldBeAdded()
    {
        $aMoney = new Money(100, new Currency('USD'));

        $newMoney = $aMoney->add(new Money(20, new Currency('USD')));

        $this->assertEquals(120, $newMoney->amount());
    }

    // ...
}