致命错误：未捕获的异常'PDOException'，消息'SQLSTATE [3D000]：无效的目录名称：1046未选择数据库'

Question

我正在尝试查询我的数据库，但一直收到致命的错误消息。

  

致命错误：未捕获异常'PDOException'，消息'SQLSTATE [3D000]：无效的目录名称：1046未选择数据库'在G：\ xampp \ htdocs \ login＆amp; register \ New_folder \ classes \ connect.php：78 Stack跟踪：＃0 G：\ xampp \ htdocs \ login＆amp; register \ New_folder \ classes \ connect.php（78）：PDOStatement-＆gt; execute（）＃1 G：\ xampp \ htdocs \ login＆amp; register \ New_folder \ core \ init.php（14）：db-＆gt; query（'SELECT`usernam ...'，Array）＃2 G：\ xampp \ htdocs \ login＆amp; register \ New_folder \ login.php（3）：require（'G ：\ xampp \ htdocs ...'）在第78行的G：\ xampp \ htdocs \login®ister\ New_folder \ classes \ connect.php中抛出＃3 {main}

有人可以请求帮助，因为我已经阅读了大量有关此问题的解决方案，但似乎都没有。

CONNECT.PHP

class db{
    private 
        $_host = 'localhost',
        $_dbname = 'mydb',
        $_username = 'root',
        $_password = 'adoloveado',
        $_pdo = NULL,
        $_query,
        $_count = 0,
        $_error = true;

private static $_instance = NULL;

private function __construct(){

    try{
        $this->_pdo = new PDO("mysql:host = {$this->_host}; dbname = {$this->_dbname}", $this->_username, $this->_password);
        $this->_pdo->setAttribute(PDO::ATTR_ERRMODE, PDO:: ERRMODE_EXCEPTION);

        if($this->_pdo){
            echo "Connected to ". strtoupper($this->_dbname) ." database successfully ...<br/>";
        }
    }catch(PDOException $e){
        die($this->_error ? $e->getMessage() : " ");
    }
}

    public function getInstance(){
    if(!isset(self::$_instance)){
        self::$_instance = new db();
    }
        return self::$_instance;
}



public function query($sql, $params = array() ){
    $this->_error = false;

    if($this->_query = $this->_pdo->prepare($sql)){
        $x = 1;
        if(count($params)){
            foreach($params as $param){
                $this->_query->bindValue($x, $param);
                $x++;
            }
        }
        if($this->_query->execute()){
            echo "Success";
         }
      }
  }
}

的init.php

<?php 

require('classes/connect.php');

db::getInstance()->query("SELECT username FROM users WHERE username = ? OR username = ?", array(

    'pingo','billy'

));