How I Use Microsoft Web Platform Installer offline?
You can find update documentation for the offline features of web platform installer here
I was able to run the following on my laptop to download the offline install cache for PHP56. It automatically downloads all possible prerequisites.
WebPICMD.exe /Offline /Products:PHP56 /Path:C:\webpioff
You can copy this folder to the server without internet access, launch web platform installer and click the options link on the bottom. You can then browse to and select the file "C:\webpioff\feeds\latest\webproductlist.xml" as your primary feed. Or, in command line:
WebPICMD.exe /Install /Products:PHP56 /XML:C:\webpioff\feeds\latest\webproductlist.xml