I'd recommend using PHP 7.0, ensure you have the latest release (7.0.25 at the time of writing) but you can use PHP 7.1 if you wish. PHP 5.6 on Windows Server has restrictions so this should be avoided.
I'd recommend using PHP 7.0, ensure you have the latest release (7.0.25 at the time of writing) but you can use PHP 7.1 if you wish. PHP 5.6 on Windows Server has restrictions so this should be avoided.