This may seem trivial, but in fact about 2 months ago some clever guys made changes in brew repository, so doing just:
brew install php71-intl
will show you error with message that such recipe doesn’t exists. Also XAMPP comes without intl extension so is there any working solution which does not require full and manual php compilation?

Fortunately, there is. There is temporary fix in another brew repo, so all you have to do is:
brew tap kyslik/homebrew-php
brew install kyslik/php/php71-intl

And this works. Really. Enjoy.