Nice plugin!
In Command_ConfirmShutdown, you create a Timer every time the command is run. If it's run twice, the countdown cannot be canceled.
Remember you can use %N to get the client's name if you use it as a variable. ie
PHP Code:
LogAction(client, -1, "%N requested a server shutdown for %i seconds.", client, shutdownTime);
Hope this helps
PM me when the timer issue is fixed.