我知道C，Java和甚至Ruby(和可能的其他编程语言)允许嵌套/内部类在主类中，允许使代码更面向对象和组织。

在PHP中，我想做一些这样的事情：

<?php
    public class User {
        public $userid;
        public $username;
        private $password;

        public class UserProfile {
            // Some code here
        }

        private class UserHistory {
            // Some code here
        }
    }
?>

这是可能在PHP吗？我该如何实现呢？如果这是不可能的，未来的PHP版本可能支持嵌套类？

嵌套类与其他类相关，有别于外部类。以Java为例：

非静态嵌套类可以访问包含类的其他成员，即使它们被声明为私有。另外，非静态嵌套类需要实例化父类的实例。

OuterClass outerObj = new OuterClass(arguments);
outerObj.InnerClass innerObj = outerObj.new InnerClass(arguments);

使用它们有几个令人信服的理由：

>这是一种逻辑分组只在一个地方使用的类的方法。

If a class is useful to only one other class, then it is logical to
relate and embed it in that class and keep the two together.

>它增加封装。

Consider two top-level classes, A and B, where B needs access to
members of A that would otherwise be declared private. By hiding class
B within class A, A’s members can be declared private and B can access
them. In addition, B itself can be hidden from the outside world.

>嵌套类可以导致更可读和可维护的代码。

A nested class usually relates to it’s parent class and together form a “package”

在PHP

你可以在没有嵌套类的PHP中有类似的行为。

如果所有你想实现的是结构/组织，如Package.OuterClass.InnerClass，PHP命名空间可能sufice。你甚至可以在同一个文件中声明多个命名空间(虽然，由于标准的自动加载功能，这可能不可取)。

namespace;
class OuterClass {}

namespace OuterClass;
class InnerClass {}

如果你想模仿其他特性，如成员可见性，需要更多的努力。

定义“包”类

namespace {

    class Package {

        /* protect constructor so that objects can't be instantiated from outside
         * Since all classes inherit from Package class, they can instantiate eachother
         * simulating protected InnerClasses
         */
        protected function __construct() {}

        /* This magic method is called everytime an inaccessible method is called 
         * (either by visibility contrains or it doesn't exist)
         * Here we are simulating shared protected methods across "package" classes
         * This method is inherited by all child classes of Package 
         */
        public function __call($method, $args) {

            //class name
            $class = get_class($this);

            /* we check if a method exists, if not we throw an exception 
             * similar to the default error
             */
            if (method_exists($this, $method)) {

                /* The method exists so now we want to know if the 
                 * caller is a child of our Package class. If not we throw an exception
                 * Note: This is a kind of a dirty way of finding out who's
                 * calling the method by using debug_backtrace and reflection 
                 */
                $trace = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 3);
                if (isset($trace[2])) {
                    $ref = new ReflectionClass($trace[2]['class']);
                    if ($ref->isSubclassOf(__CLASS__)) {
                        return $this->$method($args);
                    }
                }
                throw new \Exception("Call to private method $class::$method()");
            } else {
                throw new \Exception("Call to undefined method $class::$method()");
            }
        }
    }
}

用例

namespace Package {
    class MyParent extends \Package {
        public $publicChild;
        protected $protectedChild;

        public function __construct() {
            //instantiate public child inside parent
            $this->publicChild = new \Package\MyParent\PublicChild();
            //instantiate protected child inside parent
            $this->protectedChild = new \Package\MyParent\ProtectedChild();
        }

        public function test() {
            echo "Call from parent -> ";
            $this->publicChild->protectedMethod();
            $this->protectedChild->protectedMethod();

            echo "<br>Siblings<br>";
            $this->publicChild->callSibling($this->protectedChild);
        }
    }
}

namespace Package\MyParent
{
    class PublicChild extends \Package {
        //Makes the constructor public, hence callable from outside 
        public function __construct() {}
        protected function protectedMethod() {
            echo "I'm ".get_class($this)." protected method<br>";
        }

        protected function callSibling($sibling) {
            echo "Call from " . get_class($this) . " -> ";
            $sibling->protectedMethod();
        }
    }
    class ProtectedChild extends \Package { 
        protected function protectedMethod() {
            echo "I'm ".get_class($this)." protected method<br>";
        }

        protected function callSibling($sibling) {
            echo "Call from " . get_class($this) . " -> ";
            $sibling->protectedMethod();
        }
    }
}

测试

$parent = new Package\MyParent();
$parent->test();
$pubChild = new Package\MyParent\PublicChild();//create new public child (possible)
$protChild = new Package\MyParent\ProtectedChild(); //create new protected child (ERROR)

输出：

Call from parent -> I'm Package protected method
I'm Package protected method

Siblings
Call from Package -> I'm Package protected method
Fatal error: Call to protected Package::__construct() from invalid context

注意：

我真的不认为试图在PHP中模仿innerClasses是一个好主意。我认为代码不那么干净，可读。此外，可能有其他方法使用一个良好建立的模式，如观察者，装饰者ou组织模式实现类似的结果。有时，即使简单的继承也是足够的。