$autoload['libraries'] = array('session'); error

Hello,



Running localhost Apache with PHP Version 7.1.8 OS X Yosemite, I copied CodeIgniter into the webserver/Documents folder.



I tested it in Chrome and Safari, it works.  I started to configure it and after doing this: $autoload['libraries'] = array('session'); it broke with the following errors:


A PHP Error was encountered
Severity: Warning
Message: mkdir(): Invalid path
Filename: drivers/Session_files_driver.php
Line Number: 136
Backtrace:
File: /Library/WebServer/Documents/katana/index.php
Line: 315
Function: require_once


( ! ) Warning: Uncaught Exception: Session: Configured save path '' is not a directory, doesn't exist or cannot be created. in /Library/WebServer/Documents/katana/katana/libraries/Session/drivers/Session_files_driver.php:138 Stack trace: #0 [internal function]: CI_Session_files_driver->open('', 'ci_session') #1 /Library/WebServer/Documents/katana/katana/libraries/Session/Session.php(143): session_start() #2 /Library/WebServer/Documents/katana/katana/core/Loader.php(1281): CI_Session->__construct() #3 /Library/WebServer/Documents/katana/katana/core/Loader.php(1174): CI_Loader->_ci_init_library('Session', 'CI_', NULL, 'session') #4 /Library/WebServer/Documents/katana/katana/core/Loader.php(1037): CI_Loader->_ci_load_stock_library('Session', 'Session/', NULL, NULL) #5 /Library/WebServer/Documents/katana/katana/core/Loader.php(1082): CI_Loader->_ci_load_library('Session', NULL, NULL) #6 /Library/WebServer/Documents/katana/katana/core/Loader.php(218): CI_Loader->_ci_load_library('Session', NULL, NULL) #7 /Library/WebServer/Docum in /Library/WebServer/Documents/katana/katana/libraries/Session/drivers/Session_files_driver.php on line 138

Now what??

Thanks.