How to detect the client's operating system?

PHP
1

How to detect the client browser’s operating system?
Some websites like software downloads show the system specific pages.

By this, the user does not have to search for a compatible download or download incompatible software.

I think, parsing USER_AGENT is a difficult way unless we know a list of browsers, OS and their versions.

2

Yes, it’s difficult and only works if useragent string contains OS info.
There is a decent class on PEAR for doing it :

Check out the API