I just installed the Go Live release of the Microsoft FastCGI Extension and am using it to run PHP 5.2.3 under IIS 6 on Windows 2003 Server Standard x64 Edition (whew! what a mouthful).
I had previously run the same version of PHP as a [32-bit] ISAPI module under IIS. To accomplish this, I had to configure IIS 6 to run 32-bit application pools, which was not an issue. Once I installed the x64 version of the FastCGI extension, however, I decided to restrict my application pools once again to x64 only, just to see if this would work. Well, for the most part, it did.
Here's the catch...
I host multiple web sites in IIS, and each site requires a slightly different set of configuration parameters from the ones defined in the boilerplate php.ini file. PHP provides for this type of setup by allowing you to specify "Per-directory" values in the Windows registry. Normally, these values would be contained in the key: HKLM\Software\PHP. If running the 32-bit PHP ISAPI module in an x64 edition of Windows, as I was, then the key's location would be: HKLM\Software\Wow6432Node\PHP.
After I installed the x64 version of the FastCGI extension (and turned off 32-bit application pools in IIS), I moved my Per-directory values from their previous location in HKLM\Software\Wow6432Node\PHP to HKLM\Software\PHP (i.e., where other x64-native apps look for their settings). But PHP didn't like that. It couldn't see the custom settings. Only after I dropped the Per-directory values back into the Wow6432Node container key did PHP register them again. And that's where I left it.
Now, I recognize that I may be the only living soul on Earth trying to run PHP in this way, so I won't be surprised in the slightest if I never receive a single response to this thread :) which is just fine by me because everything is basically working. Nevertheless, I just wanted to put this out there because the behavior I described is technically not consistent with that of other x64 apps.
Please note that the fcgiext.ini file I am using to configure the FastCGI handler to work with PHP resides in my C:\WINDOWS\system32\inetsrv directory - NOT the C:\WINDOWS\SysWOW64\inetsrv directory. I mention this only to refute any suggestion that IIS may still be configured to run 32-bit application pools. If that were the case, then my FastCGI handler would not be able to process PHP files. At least that is my understanding. I could be wrong.