Fatal error: Class CI_Session_files_driver contains 1 abstract method and must...

Fatal error: Class CI_Session_files_driver contains 1 abstract method and must therefore be declared abstract or implement the remaining methods
sometimes i see this error.
 i set in my php.ini

Code:

[opcache]
;zend_extension=php_opcache.dll
; Determines if Zend OPCache is enabled
opcache.enable=0 

i am using apache2.4 and ci 3.1.8
this error is gone when i restart apache server but  i dont wanna restart apache server, how can i solve this problem?
thanks for support