Topic: Session errors
If you are getting a lot of errors in tmp/errors.log for logout.php file, it is because of the last 2 lines in it. The error lines would be like:
[31-Jul-2017 13:53:27] 0::logout.php:50:[before upgrade] session_destroy() [<a href='function.session-destroy'>function.session-destroy</a>]: Session object destruction failed
The following links have more info on it:
https://stackoverflow.com/questions/8549757/why-session-object-destruction-failed
https://stackoverflow.com/questions/3538513/detect-if-php-session-exists
https://github.com/yiisoft/yii2/commit/457abe8939b0d1b7b41c33c4c335d7d6b8ae83a6