In Linux (Ubuntu, Fedora) OS we have the PHP executable available in shell, so we can run command line php script. But in windows to do so, we have to set PHP executable as Windows path variable.
We can do that in two ways ...

1. Open the command prompt (start -> run -> cmd)
and type: set PATH=%PATH%;C:\path_to_php
here, "C:\path_to_php" is the physical folder where php.exe is located. In my case its on C:\xampp\php.

Now we can run PHP script from anywhere we want like this:
D:\>php -v
PHP 5.2.5 (cli) (built: Nov 8 2007 23:18:51)
Copyright (c) 1997-2007 The PHP Group
Zend Engine v2.2.0, Copyright (c) 1998-2007 Zend Technologies
with Zend Extension Manager v1.2.0, Copyright (c) 2003-2007, by Zend Technologies
with Xdebug v2.0.2, Copyright (c) 2002-2007, by Derick Rethans

Main problem of this method is this will affect only a single command prompt.

2. The second method will affect all command prompts. First, right click on "My Computer", then click on "Properties" - a dialog box with some tabs will appear.
Now from the tab named "Advanced" click on the button "Environment Variables". This will open the Environment Variables box. Now choose the Path variable from the system variables box and click Edit.
Append the variable value with:

;C:\path_to_php

here, "C:\path_to_php" is the physical folder where php.exe is located. In my case its on C:\xampp\php.
';' character is used to separate directories in the var, so be sure it’s present.

Thats all.