brew untap homebrew/php
brew untap exolnet/deprecated
brew cleanup

brew tap shivammathur/php
brew install shivammathur/php/php@7.1
brew services restart shivammathur/php/php@7.1
brew link --overwrite --force shivammathur/php/php@7.1

https://github.com/shivammathur/homebrew-php