Because the user agent string can easily be changed in the browser. Is there a way, other than looking up the user agent string to detect what operating system a visitor is using?
[RESOLVED] Is there a way to detect OS without relying on the user agent?
Well, you could do some JavaScript OS detection... in the end, the only way to retrieve a user's OS is to rely on the information they (or their browsers) provide.
alright then, thanks for replying 
I guess I'll add some JS, and have PHP get the info in a <noscript> for those without JS.