I was recently setting up PHP 5.6 on IIS 8.5 and hit an error.
C:Program Files (x86)\PHP\v5.6\php-cgi.exe - The FastCGI process exited unexpectedly
I had been following a guide and this one didn’t make much sense. After trying to endlessly debug this I found out that it wasn’t actually anything to do with PHP or IIS, it was a dependency issue. Thankfully, the fix for this is very simple.
In order for PHP to function under IIS 8, it needs the Visual C++ Redistributable for Visual Studio 2012 Update 4. In particular, it needs the 32-bit version. I was running under a 64 bit OS, but it seems that PHP was running as a 32-bit application and required the 32-bit redistributable. So even if you have 64 bit, download the 32-bit version of the installer.
To fix this, you can download the correct version of the installer based on the operating system that you are using from the Microsoft website here.