I am in the process on installing a IIS on Win2008, which shall power PHP websites.
I am not familiar with "Application Request Routing". Does it make sense to install this module, when the plan is only to run php-sites?
If this is a single server and if you're running PHP on IIS where it's installed and configured as per the guidelines (PHP non-threadsafe build + FastCGI) then I can't really see a reason to be adding ARR into the mix unless....
I'd leave it out until you start to hit problems that can be solved using ARR.