The Registry is where Windows and other applications store many of their configuration options. On a Windows based system, PHP's behavior can be controlled by entries in the Registry. While rare, it's interesting and worth mentioning.
Keys that affect the execution of PHP scripts within certain directories exist in the HKEY_LOCAL_MACHINE hive, and are found under HKEY_LOCAL_MACHINE\Software\PHP\Per Directory Values. For example, keys which affect scripts running in the C:\inetpub\wwwroot would be found in HKEY_LOCAL_MACHINE\Software\PHP\Per Directory Values\c\inetpub\wwwroot.
The Registry Editor program can be launched by clicking on the Start button, clicking Run, typing "regedit" and clicking OK. You may then navigate the registry using the tree structure in the left pane and edit key values in the right pane.
While Microsoft and I may feel differently about the fragility of the Registry, I feel obligated to reiterate Microsoft's Registry Mantra: "Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk."
