$ pdo以面向对象的样式返回null

Question

我目前正在使用OOP编写留言簿php练习插件，但遗憾的是我遇到了一个问题，即我的pdo连接返回null。令人难以理解的是，连接类存储在同一属性中的工作没有任何问题，只有几行之前。然而，显然，我似乎没有找到它不起作用的原因...... :(

这是db类：

class db_sql
{ 
  protected $host = 'localhost',  
            $user = 'root',
            $pw   = '',
            $dbname = 'gaestebuch';

  public $initialCn;

  static public $db_obj = null; 
  static public $cn; 

  public function __construct () 
    { 
      $this->initialCn = new PDO('mysql:host=' . $this->host . ';dbname=' . $this->dbname, $this->user, $this->pw);
      $this->initialCn->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_OBJ); 
    } 

  public static function getInstance () 
    { 
      if(self::$db_obj === null) 
        self::$cn = new db_sql; 

      return self::$db_obj; 
    } 

  public final function __clone () { } 
} 

这是用户类（仅用于继承）

class User extends db_sql
{
  protected $name = null,
            $email = null,
            $msg = null;

  public function __construct($name = 'name', $email = 'email', $msg = 'msg')
  {
    $this->name = $name;
    $this->email = $email;
    $this->msg = $msg;
  }

  public function getName(){
    return $this->name;
  }
  public function getEmail(){
    return $this->email;
  }
  public function getMsg(){
    return $this->msg;
  }

}

这是帖子类：

class Eintrag extends User
{
  public $listed_at = null;
  public static $list_decay = false,
                                $interaction = null;
  protected static $pdo;
    private static $error = false;

  public function __construct($pdo = null)
  {
    self::$pdo = new db_sql();
  }

  public static function setEintrag($user)
  {
    $name = $user->getName();
    $email = $user->getEmail();
    $msg = $user->getMsg();

    if(!empty($name) && !empty($email) && !empty($msg))
    {
            if(!self::$pdo == null)
            {
                $sql = 'INSERT INTO gaestebuch(name, email, text) VALUES(:name, :email, :text)';
                $stmt = self::$pdo->initialCn->prepare($sql);
                $stmt->execute(['name' => $name, 'email' => $email, 'text' => $msg]);
                return true;
            } else {
                self::$error = "Ein Fehler bezüglich der Datenbankverbindung ist aufgetreten.";
                self::errorListener();
                return false;
            }
    } else {
            self::$error = "Bitte füllen sie alle Felder aus.";
      self::errorListener();
      return false;
    }
  }

  public static function showPosts()
  {
    if(!self::$pdo == null)
    {
      $sql = 'SELECT * FROM gaestebuch';
      // $pdo in line 48 returns null
      $stmt = self::$pdo->initialCn->prepare($sql);
      $stmt->execute(['name' => $name, 'email' => $email, 'text' => $text, 'created_at' => $created_at]);
      $posts = $stmt->fetchAll();
      foreach($posts as $post)
      {
        echo $post->name . '( ' . $post->email . ' ) schrieb am ' . $post->created_at . ' folgendes:<br>';
        echo $post->text . '<br>';
        echo '<hr>';
        echo '<br>';
      }
      return true;
    } else {
      self::$error = "Ein Fehler bezüglich der Datenbankverbindung ist aufgetreten.";
      self::errorListener();
      return false;
    }
  }

  public static function errorListener()
  {
        if(!self::$error == false){
            echo self::$error;
            return true;
        }
  }
}

这是主要的脚本：

require_once(dirname(__DIR__).'/vars.php');

  if(isset($_GET['eintrag']))
  {
    $name = trim($_POST['name']);
    $email = trim($_POST['email']);
    $msg = trim($_POST['msg']);

    if(!filter_var($email, FILTER_VALIDATE_EMAIL) === false && !empty($name) && !empty($msg))
    {
      $user = new User($name, $email, $msg);
      $userEintrag = new Eintrag();
      $userEintrag->setEintrag($user);

    }else{
      $error = "Bitte füllen sie alle Felder richtig aus.";
      echo $error;
    }

  }

最后是我的vars.php

  define('gstl_name', 'Gaesteliste');
  define('gstl_version', '1.6.3');
  define('gstl_dir', dirname(__FILE__).'/');

  require_once(gstl_dir.'/misc/db.php');
  require_once(gstl_dir.'/misc/user.class.php');
  require_once(gstl_dir.'/misc/eintrag.class.php');

  require_once(gstl_dir.'/main/script.php');

有人知道什么是错误的和/或甚至有办法解决这个问题吗？提前感谢您的关注