php __clone需要注意的问题

class Account{
    public $balance;
   function __construct($balance){
       $this->balance=$balance;
   }
}

class Person{
  private $name;
  private $age;
  private $id;
  public $account;
  function __construct($name,$age,$account){
      $this->name=$name;
      $this->age=$age;
      $this->account=$account;
  }
  function setId($id){
    $this->id=$id;
  }
  function __clone(){
   $this->id=0;

  }
}
$p1=new Person(‘tom‘, 33, new Account(200));
$p1->setId(4);

$p2=clone $p1;
//给$p1充500 
$p1->account->balance+=500;
//结果$p2也得到了这笔钱
print $p2->account->balance; //700

function __clone(){
   $this->id=0;
   $this->account=clone $this->account;

  }