SinglnInstance.class.php

<?php
/**
 * 单例模式： 通过提供对自身共享实例的访问， 单元素设计模式用于限制特定对象只能被创建一次
 * 单例设计模式最常用于构建数据库连接对象。 
 *
 * 代码示例 - 背景介绍
 * 利用单元素设计模式连接数据库并完成CD库存表记录更新
 */
//利用单元素设计模式编写的库存记录更新表
class Singleton
{
    /**
     * @var Singleton The reference to *Singleton* instance of this class
     */
    private static $instance;
    
    /**
     * Returns the *Singleton* instance of this class.
     *
     * @return Singleton The *Singleton* instance.
     */
    public static function getInstance()
    {
        if (null === self::$instance) {
            static::$instance = new static();
        }
        
        return self::$instance;
    }

    /**
     * Protected constructor to prevent creating a new instance of the
     * *Singleton* via the `new` operator from outside of this class.
     */
    protected function __construct()
    {
    }

    /**
     * Private clone method to prevent cloning of the instance of the
     * *Singleton* instance.
     *
     * @return void
     */
    private function __clone()
    {
    }

    /**
     * Private unserialize method to prevent unserializing of the *Singleton*
     * instance.
     *
     * @return void
     */
    private function __wakeup()
    {
    }
}

class SingletonChild extends Singleton
{
}

$obj = Singleton::getInstance();
var_dump($obj === Singleton::getInstance());             // bool(true)

$anotherObj = SingletonChild::getInstance();
var_dump($anotherObj === Singleton::getInstance());      // bool(false)

var_dump($anotherObj === SingletonChild::getInstance()); // bool(true)