$stmt = $this->db->prepare("SELECT * FROM users");  
$stmt->execute();  
foreach ($stmt->fetch(PDO::FETCH_ASSOC) as $row) {
      print_r($row);
}

或：

foreach ($this->db->query("SELECT * FROM users") as $row) {  
     print_r($row)."\n";
}    

了解有关3个PDO数据库调用的详细信息...

Learn more about 3 PDO database calls here...

PDO查询

PDO Exec

PDO准备

编辑：同时检查您的 app / config / database。用于以下设置的php 文件：

Also check your app/config/database.php file for the following settings:

$active_group = 'default';  
$active_record = FALSE;  

$db['default']['hostname'] = 'YOURHOSTNAME';  
$db['default']['username'] = 'YOURUSERNAME';  
$db['default']['password'] = 'YOURPASSWORD';  
$db['default']['database'] = 'YOURDATABASE';  
$db['default']['dbdriver'] = 'mysql';  
$db['default']['dbprefix'] = '';  
$db['default']['pconnect'] = TRUE;  
$db['default']['db_debug'] = TRUE;  
$db['default']['cache_on'] = FALSE;  
$db['default']['cachedir'] = '';  
$db['default']['char_set'] = 'utf8';  
$db['default']['dbcollat'] = 'utf8_general_ci';  
$db['default']['swap_pre'] = '';  
$db['default']['autoinit'] = TRUE;  
$db['default']['stricton'] = FALSE;