I'm using Mac OS High Sierra and none of these worked for me. But after searching a lot I found one that worked!
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.
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