In response to http://shaneauckland.co.uk/2013/11/considering-what-to-test-with-phpspec.

0-usage.php

<?php

$collection = new Collection();

$collection->add('item');
$collection->add('another item');
$collection->add('item');

/*
  What would you expect here? Probably a count of 3...
  or if your collection only tracked unique items, you would expect 2. 
  This is the behaviour that you are defining and therefore it is 
  important to test it!
*/
echo count($collection);

// or 
echo $collection->count();

Collection1.php

<?php

class Collection implements \Countable
{
    private $items;

    public function __construct(array $items = [])
    {
        $this->items = $items;
    }

    public function add($item)
    {
        $this->items[] = $item;
    }

    public function count()
    {
        return count($this->items);
    }
}

CollectionSpec.php

<?php

namespace spec;

use PhpSpec\ObjectBehavior;
use Prophecy\Argument;

class CollectionSpec extends ObjectBehavior
{
    function it_is_initializable()
    {
        $this->shouldHaveType('Collection');
    }

    function it_implements_countable()
    {
        $this->shouldImplement('Countable');
    }

    function it_counts_its_items()
    {
        $this->add('item 1');
        $this->add('item 2');
        $this->add('item 3');

        $this->count()->shouldReturn(3);
    }
    
    // Using our first attempt at implementing a \Countable Collection 
    // (Collection1.php) this spec will fail...
    function it_should_only_count_unique_items()
    {
        $this->add('item');
        $this->add('item');
        $this->add('item');

        $this->count()->shouldReturn(1);
    }
}

CollectionUniqueCount.php

<?php

class Collection implements \Countable
{
    private $items;

    public function __construct(array $items = [])
    {
        $this->items = $items;
    }

    public function add($item)
    {
        if (!in_array($item, $this->items)) {
            $this->items[] = $item;
        }
    }

    public function count()
    {
        return count(array_unique($this->items));
    }
}