在构造方法中使用静态属性保留的PDO资源句柄在其他方法中无法调用？

class DB{
    protected $link = '127.0.0.1';
    protected $dbname = 'think';
    static public $DB;
    private function __construct(){
        try{
            self::$DB = new PDO("mysql:host={$this->link};dbname={$this->dbname}",'root','root');
        }catch (PDOException $e){
            die("连接出错:".$e->getMessage());
        }

        $sql = ' SELECT * FROM user WHERE id=? and username=? and email=? ';
        $stmt = self::$DB->prepare($sql);
        $stmt->execute([0=>'13',1=>'12',2=>'12']);
        echo '
';
        print_r($stmt->fetchAll(PDO::FETCH_ASSOC));
        echo $stmt->rowCount();
    }
    //静态方法，单例统一访问入口
    static public function getInstance() {
        if (is_null ( self::$DB ) || isset ( self::$DB )) {
            self::$DB = new self ();
        }
        return self::$DB;
    }
    public function Test(){
         $sql = ' SELECT * FROM user WHERE id=? and username=? and email=? ';
         $stmt = self::$DB->prepare($sql);
         $stmt->execute([0=>'13',1=>'12',2=>'12']);
         echo '
';
         print_r($stmt->fetchAll(PDO::FETCH_ASSOC));
         echo $stmt->rowCount();
    }
}
$db = DB::getInstance();
$db->Test();

我把Test方法复制在构造方法没有问题，为什么在Test方法中会出现Call to undefined method DB::prepare()？？？
求各位大神了
------解决思路----------------------
你至少应写作
class DB{
    protected $link = '127.0.0.1';
    protected $dbname = 'think';
    static public $DB;
    static public $_DB;
    private function __construct(){
        try{
            self::$_DB = new PDO("mysql:host={$this->link};dbname={$this->dbname}",'root','root');
        }catch (PDOException $e){
            die("连接出错:".$e->getMessage());
        }
    }
    //静态方法，单例统一访问入口
    static public function getInstance() {
        if (is_null ( self::$DB ) 
------解决思路----------------------
 isset ( self::$DB )) {
            self::$DB = new self ();
        }
        return self::$DB;
    }
    public function Test(){
         $sql = ' SELECT * FROM user WHERE id=? and username=? and email=? ';
         $stmt = self::$_DB->prepare($sql);
         $stmt->execute([0=>'13',1=>'12',2=>'12']);
         echo '
';
         print_r($stmt->fetchAll(PDO::FETCH_ASSOC));
         echo $stmt->rowCount();
    }
}
$db = DB::getInstance();
$db->Test();
PDO 本身已经封装的很好了，如确需要进一步封装以简化调用代码
那么应该从 PDO 继承一个 DB 类，如
class DB extends PDO {
  private static $_Instance;
  function __construct() {
	$options = array(
		PDO::MYSQL_ATTR_INIT_COMMAND => "set names gbk",
		PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
		PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
		);

    parent::__construct('mysql:dbname=test', 'root', '', $options);
  }
  //执行各种 sql 指令，并可通过参数 $param 进行扩展
  function query($sql, $param=null) {
	$res = [];
	try {
		$rs = parent::query($sql);
		do {
			if($t = $rs->fetchall()) $res[] = $t;
		}while($rs->nextRowset());
		return $res;
	} catch (PDOException $e) {
		die( "Error!: " . $e->getMessage() . "\n" );
		//  die();
	}
  }
  //查询并返回单条记录
  static function fetch($sql) {
	if(! self::$_Instance) self::$_Instance = new self;
	return self::$_Instance->query($sql)[0][0];//->fetch();
  }
  //查询并以数组方式返回多条记录
  static function fetchall($sql) {
	if(! self::$_Instance) self::$_Instance = new self;
	$res = self::$_Instance->query($sql);//->fetchall();
	if(count($res) == 1) return current($res);
  }
}
这样你就有机会这样使用了
$r = DB::fetch("select * from user where name='my'");