Q:

Determine client OS in PHP

I have two pages redirection in my index.php.  The pages are example_system_os.php and example_mobile_os.php. 
How to determine the user's operating system in PHP? 

A:

The get_browser function can be used to extract a couple of information the User-Agent HTTP header sent by the browser. 
Amongst those informations, it seems there is some data about the operating system the browser's running on -- i.e. about the client OS. 

Quoting the example given on the manual page of get_browser :
Array
(
    [browser_name_regex] => ^mozilla/5\.0 (windows; .;  windows nt 5\.1; .*rv:.*)  gecko/.* firefox/0\.9.*$
    [browser_name_pattern] => Mozilla/5.0 (Windows; ?;  Windows NT 5.1; *rv:*) Gecko/* Firefox/0.9*
    [parent] => Firefox 0.9
    [platform] => WinXP
    [browser] => Firefox
    [version] => 0.9

But note that the User-Agent HTTP header is sent by the client, which means :

It can be disabled
It can be faked (i.e. you can get any kind of garbage in it ^^ )

A:

You could parse the $_SERVER['HTTP_USER_AGENT'] string for the various platform details, but I wouldn't be confident that this was a deterministic approach, since it can easily be faked.