How to detect bots in a server side script like PHP

Humans and bots will do similar things, but bots will do things that humans don’t. Let’s try to identify those things. Before we look at behavior, let’s accept RayQuang’s comment as being useful. If a visitor has a bot’s user-agent string, it’s probably a bot. I can’t image anybody going around with “Google Crawler” (or Read more about How to detect bots in a server side script like PHP[…]