下面我们来了解如何实现一个自定义的迭代器，然后再开始慢慢理解迭代器的内部工作原理。先来看一个官方的例子

class myIterator implements Iterator {
    private $position = 0;
    private $array = array(
        "first_element",
        "second_element",
        "last_element",
    );  

    public function __construct() {
        $this->position = 0;
    }
    function rewind() {
        var_dump(__METHOD__);
        $this->position = 0;
    }
    function current() {
        var_dump(__METHOD__);
        return $this->array[$this->position];
    }
    function key() {
        var_dump(__METHOD__);
        return $this->position;
    }
    function next() {
        var_dump(__METHOD__);
        ++$this->position;
    }
    function valid() {
        var_dump(__METHOD__);
        return isset($this->array[$this->position]);
    }
}
$it = new myIterator;
foreach($it as $key => $value) {
 echo '输出键值：';
    var_dump($key, $value);
 //echo $key;
    echo "\n";
}

程序运行输出：

string(18) "myIterator::rewind"
string(17) "myIterator::valid"
string(19) "myIterator::current"
string(15) "myIterator::key"
输出键值：int(0)
string(13) "first_element"

string(16) "myIterator::next"
string(17) "myIterator::valid"
string(19) "myIterator::current"
string(15) "myIterator::key"
输出键值：int(1)
string(14) "second_element"
string(16) "myIterator::next"
string(17) "myIterator::valid"
string(19) "myIterator::current"
string(15) "myIterator::key"
输出键值：int(2)
string(12) "last_element"
string(16) "myIterator::next"
string(17) "myIterator::valid"

一般的迭代器内部需要下面的方法：
Iterator::current ― Return the current element 返回当前元素
Iterator::key ― Return the key of the current element 返回当前元素的键
Iterator::next ― Move forward to next element 移向下一个元素
Iterator::rewind ― Rewind the Iterator to the first element 重新回到第一个元素
Iterator::valid ― Checks if current position is valid 检查当前位置的有效性
如果不是很清楚迭代器的内容工作流程，可以查看下面的迭代器对数组的遍历过程：

/**
* @author 简明现代魔法 http://www.nowamagic.net
*/
class MyIterator implements Iterator
{
     private $var = array();

     public function __construct($array)
     {
         if (is_array($array)) {
            $this->var = $array;
         }
     }
     public function rewind() {
         echo "倒回第一个元素\n";
        reset($this->var);
     }
     public function current() {
        $var = current($this->var);
         echo "当前元素: $var\n";
         return $var;
     }
     public function key() {
        $var = key($this->var);
         echo "当前元素的键: $var\n";
         return $var;
     }
     public function next() {
        $var = next($this->var);
         echo "移向下一个元素: $var\n";
         return $var;
     }
     public function valid() {
        $var = $this->current() !== false;
         echo "检查有效性: {$var}\n";
         return $var;
     }
}
$values = array(1,2,3);
$it = new MyIterator($values);
foreach ($it as $k => $v) {
     print "此时键值对 -- key $k: value $v\n\n";
}

程序运行结果：

倒回第一个元素
当前元素: 1
检查有效性: 1
当前元素: 1
当前元素的键: 0
此时键值对 -- key 0: value 1

移向下一个元素: 2
当前元素: 2
检查有效性: 1
当前元素: 2
当前元素的键: 1
此时键值对 -- key 1: value 2
移向下一个元素: 3
当前元素: 3
检查有效性: 1
当前元素: 3
当前元素的键: 2
此时键值对 -- key 2: value 3
移向下一个元素: 
当前元素: 
检查有效性: 

现在就很清晰了吧？