How to use PHP iterator to implement a Fibonacci sequence

/**
* @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