PHP单例模式

单例模式：
<?php

class singleton {
    private static $instance = null;


    public function __construct()
    {
        echo 'This is not a Constructed Class';
    }

    private static function singleton(){
        if(!(self::$instance instanceof self)){
            self::$instance = new self();
        }
        return self::$instance;
    }

    public function test(){
        echo 22;
    }
}