了解OuterIterator接口PHP

我正在尝试在我的类上实现OuterIterator接口,但我不确定在方法getInnerIterator中使用什么.我知道它应该返回一个迭代器,但是我不知道在什么情况下它应该返回一个InnerIterator,尽管它已经拥有了Iterator接口的所有方法,那为什么它需要一个InnerIterator？

我完全迷失了OuterIterator

class myouterIterator implements OuterIterator 
{
    /**
    * Returns the inner Iterator for the current entry
    */
    public $data = array(1,2,3,4,5,6,7,8,9,10);
    public $position = 0;
    public function getInnerIterator()
    {
        // What code should be place here
    }
    public function rewind()
    {
        $this->position = 0;
    }
    public function valid()
    {
        return isset($this->data[$this->position]);
    }
    public function key()
    {
        return $this->position;
    }
    public function current()
    {
        return $this->data[$this->position];
    }
    public function next()
    {
        ++$this->position;
    }
}   

$OuterIt = new myouterIterator();
foreach($OuterIt as $key => $value)
{
    echo $key,' ',$value,'<br>';

}