<?php
/*
The visitor design pattern is a way of separating an algorithm from an object
structure on which it operates. As a result, we are able to add new operations to
existing object structures without actually modifying those structures.
*/

interface RoleVisitorInterface {
    public function visitUser(User $role);
    public function visitGroup(Group $role);
}

class RolePrintVisitor implements RoleVisitorInterface {
    public function visitGroup(Group $role) {
        echo 'Role: ' . $role->getName() . '<br/>';
    }

    public function visitUser(User $role) {
        echo 'Role: ' . $role->getName() . '<br/>';
    }
}

abstract class Role {

    public function accept(RoleVisitorInterface $visitor) {
        $klass = get_called_class();
        preg_match('#([^\\\\]+)$#', $klass, $extract);
        $visitingMethod = 'visit' . $extract[1];

        if (!method_exists(__NAMESPACE__ . '\RoleVisitorInterface', $visitingMethod)) {
            throw new \InvalidArgumentException("The visitor you provide cannot visit a $klass instance");
        }

        call_user_func(array($visitor, $visitingMethod), $this);
    }
}

class User extends Role {
    protected $name;

    public function __construct($name) {
        $this->name = (string)$name;
    }

    public function getName() {
        return 'User ' . $this->name . '<br/>';
    }
}

class Group extends Role {
    protected $name;

    public function __construct($name) {
        $this->name = (string)$name;
    }

    public function getName() {
        return 'Group ' . $this->name . '<br/>';
    }
}

$group = new Group('my group');
$user = new User('my user');
$visitor = new RolePrintVisitor();

$group->accept($visitor);
$user->accept($visitor);
?>