Some example code for a blog post on https://www.pmg.com/blog/

bad_traversable.php

<?php

interface Spam extends \Traversable
{
    // ...
}

final class Ham implements Spam, \IteratorAggregate
{
    public function getIterator()
    {
        return new \ArrayIterator(range(1, 10));
    }
}

extend_traversable.php

<?php

interface Spam extends \Traversable
{
    // ...
}

good_traversable.php

<?php

interface Spam extends \Traversable
{
    // ...
}

final class Ham implements \IteratorAggregate, Spam
{
    public function getIterator()
    {
        return new \ArrayIterator(range(1, 10));
    }
}

iterator_or_iteratorag.php

<?php

interface Foo extends \Iterator
{
    // ...
}

interface Bar extends \IteratorAggregate
{
    // ...
}