Or, a simpler way to do that:
<?php
print_r(posix_uname());
?>
Should print something like:
Array
(
[sysname] => Linux
[nodename] => vaio
[release] => 2.6.15-1-686
[version] => #2 Tue Jan 10 22:48:31 UTC 2006
[machine] => i686
)