Attempting to get PHP 5.6 running on Windows Server 2012 (IIS 8) I started receiving generic 500 errors.
I enabled Failed Request tracing and reviewed the trace.
The trace revealed this error:
ErrorCode The extended attributes are inconsistent. (0xff)
The problem was a missing Visual C++ runtime.
Ensure the appropriate VC++ runtime is installed for the version of PHP running.
In this case I needed the VC++ 11 runtime.