Interesting PHP question

Recently, I've come up with an interesting question that can potentially be used for interviewing candidates :)

We have such a PHP class.

class Collection implements Countable
{
    protected $items = [];

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

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

$collection = new Collection(['one', 'two', 'three']);

Which is going to be faster: count($collection) or $collection->count()?