<?php
class A{
protected static $instance;
protected static $instance_p;
public static function singleton(){
if (!static::$instance) {
$instance = new static();
static::$instance = $instance;
}
return static::$instance;
}
public static function singleton_p(){
if (!self::$instance_p) {
$instance = new self();//所有继承A的子类调用singleton_p实例化的都是A.
self::$instance_p = $instance;
}
return self::$instance_p;//instance class is A.
}
public static function who(){
echo 'Called A who, ', __CLASS__, ' ', self::class, ' ', static::class, "</br>";
}
public static function callWho(){
self::who();
static::who();
}
}
class B extends A{
protected static $instance;
protected static $instance_p;
public static function who(){
echo 'Called B who, ', __CLASS__, ' ', self::class, ' ', static::class, "</br>";
}
public static function callWho(){
parent::callWho();
self::who();
static::who();
}
}
class C extends A{
protected static $instance;
protected static $instance_p;
}
B::callWho();
//output, format messages, _CLASS_ self::class static::class
//Called A's who, A A B
//Called B's who, B B B
//Called B's who, B B B
//Called B's who, B B B
echo "</br>";
$b = B::singleton();
echo get_class($b), "<br>";//B
$c = C::singleton();
echo get_class($c), "</br>";//C
$b2 = B::singleton_p();
echo get_class($b2), "</br>";//A
$c2 = C::singleton_p();
echo get_class($c2), "</br>";//A
?>
另外我们要考虑到，B和C中的属性$instance是必须的，这样才能保证彼此互不干扰。如果B和C都去掉$instance的话，那么它们将同时继承父类A的$instance属性，
这样一来，B和C将只能持有一个共同的$instance，这个$instance到底是谁的实例，
取决于哪个类最先被实例化。比如B最先调用了singleton方法，那么C再调用singleton方法时，
是无法重新创建实例的，它只能沿用B的那个实例。