<?php
class LoveBase
{
protected static $_instance = NULL;
protected function __construct() {}
public static function app()
{
if(self::$_instance == NULL) {
self::$_instance = new self();
}
return self::$_instance;
}
public function get()
{
return 'LoveBase';
}
}
class Love extends LoveBase
{
public static function app()
{
if(self::$_instance == NULL) {
self::$_instance = new self();
}
return self::$_instance;
}
public function get()
{
return 'Love';
}
}
// Print "LoveLove" in this case(first case)
echo Love::app()->get();
echo LoveBase::app()->get();
// Print "LoveBaseLoveBase" in this case(second case)
// echo LoveBase::app()->get();
// echo Love::app()->get();
为什么两种不同的方法得出相同的结果？
比较这两种情况，该方法将在类首先实例化时起作用。
（对不起，我英语不好，希望你能说）
你定义了两个静态函数，它们都使用相同的静态变量（$_instance） - 基类的静态成员也可以通过子类访问（只要它不是私有的）。请记住，静态内容（方法和变量）是继承的，但不是克隆的。
解决方案：将成员变量设为私有，并为每个类创建一个。
class LoveBase
{
private static $_instance = NULL;
// ...
class Love extends LoveBase
{
private static $_instance = NULL;
// ...
// Print "LoveLove" in this case(first case)
//Set self::$_instance to Love object id
echo Love::app()->get();
//Static property $_instance is now already set, so LoveBase::app() won't create new self(), it will just return created and saved Love object
echo LoveBase::app()->get();
// Print "LoveBaseLoveBase" in this case(second case)
// Here is the same case, but static property $_instance filled with new self() in LoveBase class
// echo LoveBase::app()->get();
// echo Love::app()->get();