specgist.php

<?php

/*
 * Consider the following, fully tested and everything works,
 */

class BaseAdder
{
    public function add($a, $b)
    {
        return $a + $b;
    }
}

class BaseAdderSpec extends \PhpSpec\ObjectBehavior
{
    function it_adds()
    {
        $this->add(1, 2)->shouldReturn(3);
    }
}

class MyAddedSpec extends \PhpSpec\ObjectBehavior
{
    function it_is_a_added()
    {
        $this->shouldBeAnInstanceOf('BaseAdded');
    }
}

class MyAdder extends BaseAdder
{
}

/*
 * Now consider MyAdder is implemented like so, the tests all still pass
 * but the functionality is broken.
 *
 * This could happen if someone decides to override the method with some
 * new functionality but the test suite doesn't provided the safety to ensure
 * that the existing requirements of the method are still met.
 */

class MyAdder extends BaseAdder
{
    public function add()
    {
        return 'Broken!';
    }
}

/*
 * With phpunit I would create an AbstractAdderTest and extend MyAdderTest and
 * any other adder tests from it, this way the the full interface of the child
 * classes is tested.
 */