PHP snippet I wrote to detect google

Use it for whatever you want .. white hat uses? maybe clean things up to help the spiders out, make it faster to index...

black hat uses? ... oh I dunno, inject content, alter titles.. whatev.. but use at your own risk, I can say this much, it's been tested for some specific purposes to great success .. with this you can use the variable $is_google and do your thing if it's set to true.. this does not rely on user-agent because I'm told googlebot sometimes spoofs this to detect cloaking..

Actually I removed the code from here, if you want it .. PM me

Is based on the user agent?

There's a free one here to detect multiple search engines:
http://www.nes-emulator.com/x_bot.php

why not just simply use the .htaccess? it also takes less resources