I think it's not so important,but anyway it would be good to make the last PM sender value to be cleared from player's data when that last sender logs off.Cause its a little annoying when you don't notice that the guy has left,and you send the PM to someone else,then start apologizing "sorry,wrong id","oops,sorry","sorry,not for you" and so on.Also you might write something that you want to share only with this specific player who last sended you a PM,and then this something becomes public information.