PHP design pattern Visitor visitor pattern

<?php 
/** 
* 访问者模式 
* 
* 表示一个作用于某对象结构中的各元素的操作,可以在不改变各元素的类的前提下定义作用于这些元素的新操作 
* 
*/ 
abstract class Visitor 
{ 
abstract public function visitCroncreteElementA($element); 
abstract public function visitCroncreteElementB($element); 
} 
class ConcreteVisitor1 extends Visitor 
{ 
public function visitCroncreteElementA($element) 
{ 
echo get_class($element)." visit 1A<br/>"; 
} 
public function visitCroncreteElementB($element) 
{ 
echo get_class($element)." visit 1B<br/>"; 
} 
} 
class ConcreteVisitor2 extends Visitor 
{ 
public function visitCroncreteElementA($element) 
{ 
echo get_class($element)." visit 2A<br/>"; 
} 
public function visitCroncreteElementB($element) 
{ 
echo get_class($element)." visit 2B<br/>"; 
} 
} 
abstract class Element 
{ 
abstract public function accept($visitor); 
} 
class ConcreteElementA extends Element 
{ 
public function accept($visitor) 
{ 
$visitor->visitCroncreteElementA($this); 
} 
} 
class ConcreteElementB extends Element 
{ 
public function accept($visitor) 
{ 
$visitor->visitCroncreteElementB($this); 
} 
} 
class ObjectStructure 
{ 
private $_elements = array(); 
public function attach($element) 
{ 
$this->_elements[] = $element; 
} 
public function detach($element) 
{ 
if($key = array_search($element,$this->_elements) !== false) unset($this->_elements[$key]); 
} 
public function accept($visitor) 
{ 
foreach($this->_elements as $element) 
{ 
$element->accept($visitor); 
} 
} 
} 
// 
$objOS = new ObjectStructure(); 
$objOS->attach(new ConcreteElementA()); 
$objOS->attach(new ConcreteElementB()); 
$objCV1 = new ConcreteVisitor1(); 
$objCV2 = new ConcreteVisitor2(); 
$objOS->accept($objCV1); 
$objOS->accept($objCV2);