<?php
if(strcasecmp("WIN",substr(PHP_OS,0,3))==0)
	define("APE_WINDOWS", true);
else
	define("APE_WINDOWS", false);

echo "Result:".APE_WINDOWS."\n";

echo "Defined?:".defined("APE_WINDOWS")."\n";
?>

Windows output:

C:\Program Files (x86)\Apache Software Foundation\Apache2.2\htdocsdonny>php -v
PHP 5.3.6 (cli) (built: Mar 17 2011 10:37:07)
Copyright (c) 1997-2011 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2011 Zend Technologies

C:\Program Files (x86)\Apache Software Foundation\Apache2.2\htdocsdonny>php -q test_win.php
Result:1
Defined?:1

Linux output:

PHP 5.3.6 (cli) (built: Mar 19 2011 08:30:07) 
Copyright (c) 1997-2011 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2011 Zend Technologies
Result:
Defined?:1