[CodeIgniter][Error] Class CI_Session_files_driver contains 1 abstract method and... 하는 에러가 날 때

A PHP Error was encountered

Severity: Error

Message: Class CI_Session_files_driver contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (SessionHandlerInterface::write)

Filename: drivers/Session_files_driver.php

Line Number: 49

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이 에러가 날 때는

그냥 아파치 서버를 재시작하면 해결된다..