Looking for  a guide on how to enable and use PHP extensions on Windows? Richard Davey has a quick guide  over at Core PHP:

Have you ever run into this problem when using PHP on Windows: You have uncommented an extension in your php.ini, you’ve checked that the DLL file exists, and is actually present, ready and waiting in your extensions folder, but when you start Apache you get a ‘could not load module’ error, claiming that the DLL you KNOW exists, isn’t there.

Link: A Guide to using PHP 5 Extensions on Windows - Core PHP