Sessions + logging in from only one location

[eluser]Jaketoolson[/eluser]
The 'hash' method is the same one I've been using since 2004 on all sites requiring an authentication system. I've added a few other security checks to this as on most sites I only allow one login instance from the user at a time. This is so that users who have purchased our product/services don't give their information to others because if they did and someone else logged in with their credentials it would log them out... so it creates a huge nuisance on their end really (which is what I want - a deterrent)