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？

<?phpclass 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);        $stmt->execute([0=>'13',1=>'12',2=>'12']);        echo '<pre class="brush:php;toolbar:false">';        print_r($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);         $stmt->execute([0=>'13',1=>'12',2=>'12']);         echo '<pre class="brush:php;toolbar:false">';         print_r($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

回复讨论(解决方案)

$DB->prepare没有prepare方法，说明$DB可能没有被实例化，在实例化之后打印DB类里吗的$DB变量看看

在构造函数中，self::$DB 是 PDO 对象，所以有prepare 方法

在 getInstance 方法中，你又 self::$DB = new self (); 把 self::$DB 赋值为 DB 对象，所以就没有了 prepare 方法

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::$DB ) || isset ( self::$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);         $stmt->execute([0=>'13',1=>'12',2=>'12']);         echo '<pre class="brush:php;toolbar:false">';         print_r($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'");

        if (!self::$DB instanceof self) {            self::$DB = new self ();        }        return self::$DB;

多谢@xuzuning