Iteratorをimplementsする奴は情弱。IteratorAggregateを使え ref: http://qiita.com/Hiraku/items/14722922441f9ed3fbbb

file0.php

<?php
class A {
  public $hoge = 1;
  public $fuga = 2;
  private $pri = 3;
  protected $pro = 4;
}

$a = new A;

foreach ($a as $key => $val) {
  var_dump($key, $val);
}

file1.txt

string(4) "hoge"
int(1)
string(4) "fuga"
int(2)

file2.php

interface Iterator extends Traversable
{
  function current();
  function key();
  function next();
  function rewind();
  function valid();
}

file3.php

//冒頭のFabienさんの記事から丸ごと引用
class Foo implements Iterator
{
    protected $attributes = array();

    function __construct(array $arr)
    {
        $this->attributes = $arr;
    }

    function rewind()
    {
        reset($this->attributes);
    }

    function current()
    {
        return current($this->attributes);
    }

    function key()
    {
        return key($this->attributes);
    }

    function next()
    {
        return next($this->attributes);
    }

    function valid()
    {
        return false !== current($this->attributes);
    }
}

file4.php

interface IteratorAggregate extends Traversable
{
  function getIterator();
}

file5.php

class Foo implements IteratorAggregate
{
    protected $attributes = array();

    function __construct(array $arr)
    {
        $this->attributes = $arr;
    }

    function getIterator()
    {
        return new ArrayIterator($this->attributes);
    }
}

file6.php

class Foo implements IteratorAggregate
{
    protected $attributes = array();

    function __construct(array $arr)
    {
        $this->attributes = $arr;
    }

    function getIterator()
    {
        foreach ($this->attributes as $key => $val) {
            yield $key => $val;
        }
    }
}