I've built the pcntl exension to work with PHP 7.1 64-bit on Windows. The extension is compatible with Unix pcntl api, but is not functional yet as it will take much time to convert Unix's signals to Windows procees model.
However, you can add it to your PHP's extension to overcome the extension-missing stopper.
Thread Safe - your PHP's version doesn't have -nts-:
Non Thread Safe: