Not sure if this is the right place to comment, but I have this issue on Windows (git-bash shell) still (or again). This is the last couple of lines of arc diff --trace:

Another +1 from me. Would it make sense to just ask the submitter for a revision to use as the base?

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.