Operating System Sniffer

Does anybody know of a variable that tells you the users Operating System... preferably in PHP but if it only exists in JavaScript thats fine also. I know PHP has the getbrowser() function and the $HTTP_USER_AGENT to get what browser they have so I was assuming there is something similar for Operating System (Mac or Windows).

no as far as I know it is in the $HTTP_USER_AGENT

really, I never even thought to see what $HTTP_USER_AGENT printed out... I just used a snippet of code to see if it was a IE or Netscape browser, thanks

it should print out something similar to this

"Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)"