The Windows Registry is simply a hierarchically structured database in which the Windows operating system and other applications store important configuration information. All sorts of data is stored in the registry: user profiles, file-type associations, hardware information, serial numbers, and more. With functions available from the win32std extension, your Windows-based PHP applications can access the registry too. The win32std extension can be downloaded as a pre-compiled library from downloads.php.net/pierre/. Simply drop it into your PHP's ext folder, update your php.ini file, and restart Apache if necessary.
The article talks a bit about the general layout of the registry, the main five keys and the encouragement of making a sandbox registry to perform your operations on rather than taking a chance. He shows how to use the extension's functions to read from the registry and write back out to a certain key. Their example application performs a check on the registry to see if a USB dongle is attached to the system based on a key being set and the device ID.