PHP to research why people stop play game

I have a online game it is massive one (MMORPG), in the lasts days the number of players start decrease, I am freak out to get the reason, so I decided send email for players that are not active any more, to invite them to play again, I will offer a prize like mousepad of company if they reply a research questions, or if they refuse they say the reason, but I do not have the critery do define what is a player that is not active any more. I need someone help me define critery for player not active, than develop php to email those people, them make php to check if number of not active player decrease after this email contact.