PHP universal database connection method

<?php class DB {
    const HOST=&#39;127.0.0.1&#39;;
    const USER=&#39;root&#39;;
    const PASS=&#39;root&#39;;
    const DATA=&#39;mooc&#39;;
    static public $_instance;
    private function __construct() {
    mysql_connect(DB::HOST,DB::USER,DB::PASS);
    mysql_set_charset(&#39;utf8&#39;);
    mysql_select_db(DB::DATA);
}
public static function getInstance() {
    if(!self::$_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);
}
?>