Memcached session storage driver error

Hello. Thanks for your Work.

I tried to use Memcached instead of database for Session storage (on CodeIgniter 3.1.6):

PHP Code:

$config['sess_driver'] = 'memcached';
$config['sess_cookie_name'] = 'wsc_sessions';
$config['sess_expiration'] = 7200;
$config['sess_save_path'] = 'localhost:11211';
$config['sess_match_ip'] = FALSE;
$config['sess_time_to_update'] = 7200;
$config['sess_regenerate_destroy'] = FALSE; 

But there was an error:
Warning: Uncaught Error: Class 'Memcached' not found in /system/libraries/Session/drivers/Session_memcached_driver.php:108 Stack trace:
#0 [internal function]: CI_Session_memcached_driver->open('/var/lib/php/se...', 'wsc_sessions')
#1 /system/libraries/Session/Session.php(143): session_start()
#2 /system/core/Loader.php(1279): CI_Session->__construct()
#3 /system/core/Loader.php(1172): CI_Loader->_ci_init_library('Session', 'CI_', NULL, 'session')
#4 /system/core/Loader.php(1037): CI_Loader->_ci_load_stock_library('Session', 'Session/', NULL, NULL)
#5 /system/core/Loader.php(1083): CI_Loader->_ci_load_library('Session', NULL, NULL)
#6 /system/core/Loader.php(218): CI_Loader->_ci_load_library('Session', NULL, NULL)
#7 /application/controllers/Wifi.php(11): CI_ in /system/libraries/Session/drivers/Session_memcached_driver.php on line 108 Fatal error: session_start(): Failed to initialize storage module: user (path: /var/lib/php/sessions) in /system/libraries/Session/Session.php on line 143