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

在构造方法中使用静态属性保存的PDO资源句柄在其他方法中无法调用？？？ ?php classDB{ protected$link='127.0.0.1'; protected$dbname='think'; staticpublic$DB; privatefunction__construct(){ try{ self::$DB=newPDO("mysql:host={$this-link};dbname={$t

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【本文来自鸿网互联 (http://www.68idc.cn)】 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'");