<?php
function getOS($userAgent) {
  // Create list of operating systems with operating system name as array key 
	$oses = array (
		'Windows 3.11' => 'Win16',
		'Windows 95' => '(Windows 95)|(Win95)|(Windows_95)', // Use regular expressions as value to identify operating system
		'Windows 98' => '(Windows 98)|(Win98)',
		'Windows 2000' => '(Windows NT 5.0)|(Windows 2000)',
		'Windows XP' => '(Windows NT 5.1)|(Windows XP)',
		'Windows 2003' => '(Windows NT 5.2)',
		'Windows NT 4.0' => '(Windows NT 4.0)|(WinNT4.0)|(WinNT)|(Windows NT)',
		'Windows ME' => 'Windows ME',
		'Open BSD'=>'OpenBSD',
		'Sun OS'=>'SunOS',
		'Linux'=>'(Linux)|(X11)',
		'Macintosh'=>'(Mac_PowerPC)|(Macintosh)',
		'QNX'=>'QNX',
		'BeOS'=>'BeOS',
		'OS/2'=>'OS/2',
		'Search Bot'=>'(nuhk)|(Googlebot)|(Yammybot)|(Openbot)|(Slurp/cat)|(msnbot)|(ia_archiver)'
	);

	foreach($oses as $os=>$pattern){ // Loop through $oses array
    // Use regular expressions to check operating system type
		if(eregi($pattern, $userAgent)) { // Check if a value in $oses array matches current user agent.
			return $os; // Operating system was matched so return $oses key
		}
	}
	return 'Unknown'; // Cannot find operating system so return Unknown
}
?>

<?php
echo getOS($_SERVER['HTTP_USER_AGENT']);
?>