<?php

class Single{
public static $instance = null;

//防止子类扩大权限，使外部可以new对象
final protected function __construct(){

}

public static function getInstance(){
if(self::$instance === null){
self::$instance = new self();
}
return self::$instance;
}

//防止clone
final protected function __clone(){

}
}

$s1 = Single::getInstance();
$s2 = Single::getInstance();
if($s1 === $s2){
echo "s1和s2是一个对象";
}else{
echo "s1和s2不是一个对象";
}

php设计模式（2）-单例模式