PHP类函数__construct传递属性问题

Question

在对php课程进行研究之后，我一直坚持为什么我的php课程只会以一种方式工作而不是我期望的。我希望能够在创建类的新实例时传递ini文件位置。 以下代码抛出parse_ini_file打开文件错误。

class MyPDO extends PDO {
    public function __construct($file){
      //print $file returns '/usr/local/www/config/db.ini'
      if (!$settings = parse_ini_file($file, TRUE)) throw new 
      exception('Unable to open ' . $file . '.');

      $dns = $settings['database']['driver'] . ':host=' . 
      $settings['database']['host'];

      parent::__construct($dns, $settings['database']['username'], 
      $settings['database']['password']);
    }
}

try {
    $conn_login = new MyPDO('/usr/local/www/config/db.ini');
    $conn_login->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
}
catch(PDOException $e){
    echo "Connection failed: " . $e->getMessage();
}

然而，以下内容将按预期工作;

class MyPDO extends PDO{
    public function __construct($file){
        $file = '/usr/local/www/config/db.ini'; //added to make work
        if (!$settings = parse_ini_file($file, TRUE)) throw new 
        exception('Unable to open ' . $file . '.');

        $dns = $settings['database']['driver'] . ':host=' . 
        $settings['database']['host'];

        parent::__construct($dns, $settings['database']['username'], 
        $settings['database']['password']);
    }
}

try {
    $conn_login = new MyPDO('/usr/local/www/config/db.ini');
    $conn_login->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
}
catch(PDOException $e){
    echo "Connection failed: " . $e->getMessage();
}