I think you're asking:
Q: How can a web server identify the client platform?
A: There's no SURE way to do this.
Your best bet is to look at the http header from the client request. Specifically, look at the "user-agent".
Here's a link with a similar question:
Is it possible to identify Android apps from http headers?
Here are three examples:
http://www.askdavetaylor.com/detect_apple_iphone_user_web_site_server.html
1) IE, from a Windows PC:
HTTP_USER_AGENT=Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 2.0.50727)
2) An Apple iPhone with Safari:
HTTP_USER_AGENT=Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en)
AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1C25 Safari/419.3
3) An Android phone:
Moxilla/5.0 (Linux; U; Android 2.2; en-us; T-Mobile G2 Build/FRF91) AppleWebKit/533.1(KHTML, like Gecko) Version/4.0 Mobile Safari/533.1