How to get the OS on which PHP is running?

PHP has many predefined constants that are often useful.

Here, PHP_OS is the one you are looking for.

For instance, on my current machine, this code :

var_dump(PHP_OS);

Gives :

string 'Linux' (length=5)

You have some examples and comparisons with what the php_uname function can get you on the manual page of php_uname ; for instance (quoting) :

<?php
echo php_uname();
echo PHP_OS;

/* Some possible outputs:
Linux localhost 2.4.21-0.13mdk #1 Fri Mar 14 15:08:06 EST 2003 i686
Linux

FreeBSD localhost 3.2-RELEASE #15: Mon Dec 17 08:46:02 GMT 2001
FreeBSD

Windows NT XN1 5.1 build 2600
WINNT
*/

if (strtoupper(substr(PHP_OS, 0, 3)) === 'WIN') {
    echo 'This is a server using Windows!';
} else {
    echo 'This is a server not using Windows!';
}

That page also says :

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.

Just bear in mind that PHP_OS actually contains the platform on which PHP was built. This may not be the same platform as that on which it is deployed. Therefore php_uname('s') is more reliable.