PHP万能的连接数据库

<?php
    class DB{
        const HOST='127.0.0.1';
        const USER='root';
        const PASS='root';
        const DATA='mooc';
        static public $_instance;
        
        private function __construct(){
            mysql_connect(DB::HOST,DB::USER,DB::PASS);
            mysql_set_charset('utf8');
            mysql_select_db(DB::DATA);
        }
        
        public static function getInstance(){
            if(!self::$_instance instanceof self){
                self::$_instance=new self;
            }
            return self::$_instance;
        }
    }
    
    $model=DB::getInstance();
    //连接数据库，并且测试
    $sql="select * from user";
    $res=mysql_query($sql);
    while($row=mysql_fetch_assoc($res)){
        var_dump($row);
    }
?>