Thank you for the issue report. We've already identified the issue, it's actually related to Microsoft Web Platform Installer. I've reported the issue to Microsoft and we are waiting for the fix.
To provide more information, it's related to Web Platform Installer 5.1. If you'd uninstall it and then try to install PHP 7.1 with PHP Tools, we will install WebPI 5.0 and then install PHP 7.1 and everything will work. Also other PHP versions would install fine even on WebPI 5.1.
Thank you for understanding