After I downloaded the installer and installed XAMPP in c:\xampp I downloaded one of my development builds of the php extension. Please note that this is a development build I've used for simple testing for myself, and I provide it AS IS WITHOUT ANY WARRANTIES so use it on your own risk (and not in production ;) ).
To "install" the extension, extract the zipfile and execute the following commands from within that directory:
copy php_couchbase.dll c:\xampp\php\ext copy libcouchbase.dll c:\xampp\php copy libcouchbase.dll c:\xampp\apache\bin
Use the XAMPP Control panel to edit the php.ini file, and enable the extension by adding:
(You may also change other tunables. See couchbase.ini for a full list).
(Re)start the Apache server from the XAMPP Control panel, and start writing code using the PHP extension. You'll find a description of the API in the file couchbase-api.php