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？

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<br /><br />class DB{<br />    protected $link = '127.0.0.1';<br />    protected $dbname = 'think';<br />    static public $DB;<br />    private function __construct(){<br />        try{<br />            self::$DB = new PDO("mysql:host={$this->link};dbname={$this->dbname}",'root','root');<br />        }catch (PDOException $e){<br />            die("连接出错:".$e->getMessage());<br />        }<br /><br />        $sql = ' SELECT * FROM user WHERE id=? and username=? and email=? ';<br />        $stmt = self::$DB->prepare($sql);<br />        $stmt->execute([0=>'13',1=>'12',2=>'12']);<br />        echo '<pre class="brush:php;toolbar:false">';<br />        print_r($stmt->fetchAll(PDO::FETCH_ASSOC));<br />        echo $stmt->rowCount();<br />    }<br />    //静态方法，单例统一访问入口<br />    static public function getInstance() {<br />        if (is_null ( self::$DB ) || isset ( self::$DB )) {<br />            self::$DB = new self ();<br />        }<br />        return self::$DB;<br />    }<br />    public function Test(){<br />         $sql = ' SELECT * FROM user WHERE id=? and username=? and email=? ';<br />         $stmt = self::$DB->prepare($sql);<br />         $stmt->execute([0=>'13',1=>'12',2=>'12']);<br />         echo '<pre class="brush:php;toolbar:false">';<br />         print_r($stmt->fetchAll(PDO::FETCH_ASSOC));<br />         echo $stmt->rowCount();<br />    }<br />}<br />$db = DB::getInstance();<br />$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{<br />    protected $link = '127.0.0.1';<br />    protected $dbname = 'think';<br />    static public $DB;<br />    static public $_DB;<br />    private function __construct(){<br />        try{<br />            self::$_DB = new PDO("mysql:host={$this->link};dbname={$this->dbname}",'root','root');<br />        }catch (PDOException $e){<br />            die("连接出错:".$e->getMessage());<br />        }<br />    }<br />    //静态方法，单例统一访问入口<br />    static public function getInstance() {<br />        if (is_null ( self::$DB ) <br><font color='#FF8000'>------解决思路----------------------</font><br> isset ( self::$DB )) {<br />            self::$DB = new self ();<br />        }<br />        return self::$DB;<br />    }<br />    public function Test(){<br />         $sql = ' SELECT * FROM user WHERE id=? and username=? and email=? ';<br />         $stmt = self::$_DB->prepare($sql);<br />         $stmt->execute([0=>'13',1=>'12',2=>'12']);<br />         echo '<pre class="brush:php;toolbar:false">';<br />         print_r($stmt->fetchAll(PDO::FETCH_ASSOC));<br />         echo $stmt->rowCount();<br />    }<br />}<br />$db = DB::getInstance();<br />$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 {<br />  private static $_Instance;<br />  function __construct() {<br />	$options = array(<br />		PDO::MYSQL_ATTR_INIT_COMMAND => "set names gbk",<br />		PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,<br />		PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,<br />		);<br /><br />    parent::__construct('mysql:dbname=test', 'root', '', $options);<br />  }<br />  //执行各种 sql 指令，并可通过参数 $param 进行扩展<br />  function query($sql, $param=null) {<br />	$res = [];<br />	try {<br />		$rs = parent::query($sql);<br />		do {<br />			if($t = $rs->fetchall()) $res[] = $t;<br />		}while($rs->nextRowset());<br />		return $res;<br />	} catch (PDOException $e) {<br />		die( "Error!: " . $e->getMessage() . "\n" );<br />		//  die();<br />	}<br />  }<br />  //查询并返回单条记录<br />  static function fetch($sql) {<br />	if(! self::$_Instance) self::$_Instance = new self;<br />	return self::$_Instance->query($sql)[0][0];//->fetch();<br />  }<br />  //查询并以数组方式返回多条记录<br />  static function fetchall($sql) {<br />	if(! self::$_Instance) self::$_Instance = new self;<br />	$res = self::$_Instance->query($sql);//->fetchall();<br />	if(count($res) == 1) return current($res);<br />  }<br />}<br />

这样你就有机会这样使用了
$r = DB::fetch("select * from user where name='my'");