如何用PHP迭代器来实现一个斐波纳契数列

/**
* @author 简明现代魔法 http://www.nowamagic.net
*/
class Fibonacci implements Iterator { 
 private $previous = 1; 
 private $current = 0; 
 private $key = 0; 
 
 public function current() { 
 return $this->current; 
 } 
 
 public function key() { 
 return $this->key; 
 } 
 
 public function next() { 
// 关键在这里
// 将当前值保存到 $newprevious
 $newprevious = $this->current; 
// 将上一个值与当前值的和赋给当前值
 $this->current += $this->previous; 
// 前一个当前值赋给上一个值
 $this->previous = $newprevious; 
 $this->key++; 
 } 
 
 public function rewind() { 
 $this->previous = 1; 
 $this->current = 0; 
 $this->key = 0; 
 } 
 
 public function valid() { 
 return true; 
 } 
} 
$seq = new Fibonacci; 
$i = 0; 
foreach ($seq as $f) { 
 echo "$f "; 
 if ($i++ === 15) break; 
}

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610