A PHP Error was encountered
Severity: Warning
Message: mkdir(): Invalid path
Filename: drivers/Session_files_driver.php
Line Number: 117
Backtrace:
File: /Users/Elsa/Sites/application/controllers/Login.php
Line: 22
Function: __construct
File: /Users/Elsa/Sites/index.php
Line: 292
Function: require_once

An uncaught Exception was encountered
Type
 
: Exception
Message: Session: Configured save path '' is not a directory, doesn't exist or cannot be created.
Filename: /Users/Elsa/Sites/system/libraries/Session/drivers/Session_files_driver.php
Line Number: 119
Backtrace:
File: /Users/Elsa/Sites/application/controllers/Login.php
Line: 22
Function: __construct
File
 
: /Users/Elsa/Sites/index.php
Line: 292
Function: require_once

$config['sess_driver'] = 'files';
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 7200;
$config['sess_save_path'] = NULL;
$config['sess_match_ip'] = FALSE;
$config['sess_time_to_update'] = 300;
$config['sess_regenerate_destroy'
 
] = FALSE;

因為這一條

$config['sess_save_path'] = NULL;

的session儲存的地址被定義為NULL。正確的做法應該是給他寫一個儲存的路徑。建議是用tmp文件，會清空暫存的session。
然而我新建了一個folder名叫session_repository，然後把我的路徑指定為（我的作業系統是mac）：

$config['sess_save_path'] = '/Users/xxxx/Sites/session_repository';

於是這個問題就解決了，不過跳轉到下一個問題：
Unable to locate the specified class: Session.php

然而我按照網上的資料，把autoload.php裡面的內容改成如下也無法解決：

$autoload['drivers'] = array('session');

未完待續……