php 单例模式

<?php

class DB{
    protected $name = "DB";
    private static $pattern;
    private $db;

    //数据库连接参数
    const HOSTNAME = "127.0.0.1";
    const USERNAME = "root";
    const PASSWORD = "root";
    const DBNAME   = "wwwlwcom";
        //构造函数
    private function __construct(){
        //连接数据库
        $this->db = mysqli_connect(self::HOSTNAME,self::USERNAME,self::PASSWORD,self::DBNAME);
    }
        //魔术克隆方法
    protected function __clone()
    {
        // TODO: Implement __clone() method.
    }

    public static function getPattern(){
        if(!(self::$pattern instanceof self)){
            self::$pattern = new self();
        }
        return self::$pattern;
    }
    //数据库查询
    public function getInfo(){
        $sql = "select * from lw_fiction";
        $res = mysqli_query($this->db,$sql);
        while($row = mysqli_fetch_array($res)){
            echo ($row['cover']." <br/>".$row['title']." <br/>".$row['author']);
        }
        mysqli_free_result($res);
    }
}
$demo = DB::getPattern();
$demo->getInfo();