访问者模式（Visitor）

模式架构简单代码实现示例：

<?php
interface Visitor { // 抽象访问者角色
    public function visitConcreteElementA(ConcreteElementA $elementA);
    public function visitConcreteElementB(concreteElementB $elementB);
}

interface Element { // 抽象节点角色
    public function accept(Visitor $visitor);
}

class ConcreteVisitor1 implements Visitor { // 具体的访问者1
    public function visitConcreteElementA(ConcreteElementA $elementA) {}
    public function visitConcreteElementB(ConcreteElementB $elementB) {}
}
class ConcreteVisitor2 implements Visitor { // 具体的访问者2
    public function visitConcreteElementA(ConcreteElementA $elementA) {}
    public function visitConcreteElementB(ConcreteElementB $elementB) {}
}
class ConcreteElementA implements Element { // 具体元素A
    private $_name;
    public function __construct($name) { $this->_name = $name; } 
    public function getName() { return $this->_name; }
    public function accept(Visitor $visitor) { // 接受访问者调用它针对该元素的新方法
        $visitor->visitConcreteElementA($this);
    }
}
class ConcreteElementB implements Element { // 具体元素B
    private $_name; 
    public function __construct($name) { $this->_name = $name;}
    public function getName() { return $this->_name; }
    public function accept(Visitor $visitor) { // 接受访问者调用它针对该元素的新方法
        $visitor->visitConcreteElementB($this);
    }
}
class ObjectStructure { // 对象结构 即元素的集合
    private $_collection; 
    public function __construct() { $this->_collection = array(); } 
    public function attach(Element $element) {
        return array_push($this->_collection, $element);
    }
    public function detach(Element $element) {
        $index = array_search($element, $this->_collection);
        if ($index !== FALSE) {
            unset($this->_collection[$index]);
        }
        return $index;
    }
    public function accept(Visitor $visitor) {
        foreach ($this->_collection as $element) {
            $element->accept($visitor);
        }
    }
}
// client
$elementA = new ConcreteElementA("ElementA");
$elementB = new ConcreteElementB("ElementB");
$elementA2 = new ConcreteElementB("ElementA2");
$visitor1 = new ConcreteVisitor1();
$visitor2 = new ConcreteVisitor2();
$os = new ObjectStructure();
$os->attach($elementA);
$os->attach($elementB);
$os->attach($elementA2);
$os->detach($elementA);
$os->accept($visitor1);
$os->accept($visitor2);