PDO代码和PHP未定义错误

class dbConn{
public $dbname='database';
private $dbhost='localhost';
private $dbusername='username';
private $dbpassword='password';


protected static $db;
private function __construct() {

try {

self::$db = new PDO('mysql:host='.$dbhost.';dbname='.$dbname,$dbusername,$dbpassword );
self::$db->setAttribute( PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
}
catch (PDOException $e) {

echo "Connection Error: " . $e->getMessage();
}

}


public static function getConnection() {


if (!self::$db) {

new dbConn();
}


return self::$db;
}

}




class Mysql{

    function showTables(){
                $db = dbConn::getConnection();

                $query=$db->query("show tables");

                $fetchArray = $query->fetchAll(PDO::FETCH_ASSOC);
                foreach($fetchArray as $index=>$val) {
                            echo $val;
                          }
    }
}

$obj=new Mysql;
$obj->showTables();

我面临的问题是,如果我直接将数据库登录详细信息用于pdo语句,如下所示：

self::$db = new PDO( 'mysql:host=localhost;dbname=databasename','root','pass');

我没有任何问题.但是,如果我像上面所说的那样使用变量并将值存储到它们中.

Notice: Undefined variable: dbhost 

Notice: Undefined variable: dbname 

Notice: Undefined variable: dbusername 

Notice: Undefined variable: dbpassword

以下错误来了,另外在其他类中也无法连接到数据库.

谁能告诉我这个问题是什么？