PHP单例模式

<?php
/**
* 
*/
class DB
{
    public $link;

    private static $db;//看当前这个类有没有被实例化

    private function __construct($host,$user,$pwd,$dbname){
        $this->link = mysqli_connect($host,$user,$pwd,$dbname);
    }

    private function __clone(){}

    //单例模式的核心思想：自己调用自己
    public static function connect_mysql($host,$user,$pwd,$dbname){
        if(!(self::$db instanceof self)){
            self::$db = new self($host,$user,$pwd,$dbname);
        }
        return self::$db;
    }

    

}

$obj = DB::connect_mysql('127.0.0.1','root','root','1812a');
print_r($obj);