There should be a Windows installer (probably .exe or .msi) for Arcanist. It could try to find a PHP install (e.g. look where windows.php.net installs to by default), and otherwise install one.
For now this might be "Details" for Wikimedia, because we still don't have real users relying on Arc for code contributions. We know that many new contributors using Windows have a hard time setting up git-review for Gerrit. We still don't know how this situation compares with Arc.
My two cents: a complete install of PHP from windows.php.net is around 45 Mb, which is barely noticeable. It should be possible to install a known good PHP version privately along with the script and use that one. I would prefer that to getting a system-wide installation of PHP when installing Arcanist. This would also solve the problem with a possible existing install that is too old or broken.
@epriestley and @mattflaschen I have created a windows installer here https://github.com/paladox/Arcanist-installer-for-windows
I'm wondering @epriestley if you would like to take control of it for windows users. It makes it easy to install plus uses a free open source installer that git for windows also use.