php_uname returns a description of the operating system PHP is running on. This is the same string you see at the very top of the phpinfo output. For the name of just the operating system, consider using the PHP_OS constant, but keep in mind this constant will contain the operating system PHP was built on.
On some older UNIX platforms, it may not be able to determine the current OS information in which case it will revert to displaying the OS PHP was built on. This will only happen if your uname() library call either doesn't exist or doesn't work.
mode is a single character that defines what information is returned:
'a': This is the default. Contains all modes in the sequence "s n r v m".
's': Operating system name. eg. FreeBSD.
'n': Host name. eg. localhost.example.com.
'r': Release name. eg. 5.1.2-RELEASE.
'v': Version information. Varies a lot between operating systems.
'm': Machine type. eg. i386.
Returns the description, as a string.