如何在PHP中通过类实现访问者模式的详细教程？

// Visitor 接口
interface Visitor {
    public function visitConcreteElementA(ConcreteElementA $element);
    public function visitConcreteElementB(ConcreteElementB $element);
}

// 具体 Visitor 类
class ConcreteVisitor1 implements Visitor {
    public function visitConcreteElementA(ConcreteElementA $element) {
        return "ConcreteVisitor1 visiting ConcreteElementA\n";
    }

    public function visitConcreteElementB(ConcreteElementB $element) {
        return "ConcreteVisitor1 visiting ConcreteElementB\n";
    }
}

class ConcreteVisitor2 implements Visitor {
    public function visitConcreteElementA(ConcreteElementA $element) {
        return "ConcreteVisitor2 visiting ConcreteElementA\n";
    }

    public function visitConcreteElementB(ConcreteElementB $element) {
        return "ConcreteVisitor2 visiting ConcreteElementB\n";
    }
}

// Element 接口
interface Element {
    public function accept(Visitor $visitor);
}

// 具体 Element 类
class ConcreteElementA implements Element {
    public function accept(Visitor $visitor) {
        return $visitor->visitConcreteElementA($this);
    }

    public function operationA() {
        return "ConcreteElementA operation\n";
    }
}

class ConcreteElementB implements Element {
    public function accept(Visitor $visitor) {
        return $visitor->visitConcreteElementB($this);
    }

    public function operationB() {
        return "ConcreteElementB operation\n";
    }
}

// 对象结构
class ObjectStructure {
    private $elements = [];

    public function attach(Element $element) {
        $this->elements[] = $element;
    }

    public function detach(Element $element) {
        $this->elements = array_filter($this->elements, function($e) use ($element) {
            return $e !== $element;
        });
    }

    public function accept(Visitor $visitor) {
        $result = '';
        foreach ($this->elements as $element) {
            $result .= $element->accept($visitor);
        }
        return $result;
    }
}

// 客户端代码
$objectStructure = new ObjectStructure();
$objectStructure->attach(new ConcreteElementA());
$objectStructure->attach(new ConcreteElementB());

$visitor1 = new ConcreteVisitor1();
echo $objectStructure->accept($visitor1);

$visitor2 = new ConcreteVisitor2();
echo $objectStructure->accept($visitor2);