These steps should have been mentioned in the prerequisites of the Laravel Installation Guide, since I'm surely not the only person trying to get Laravel running on macOS.
Install Mcrypt using Homebrew
# PHP 7
brew install php70-mcrypt
# PHP 5
brew install php56-mcrypt --without-homebrew-php
Add this line to /private/etc/php.ini
:
# PHP 7
extension="/usr/local/Cellar/php70-mcrypt/7.0.x/mcrypt.so"
# PHP 5
extension="/usr/local/Cellar/php56-mcrypt/5.6.x/mcrypt.so"
You are ready to go!
Upgrade Mcrypt using Homebrew
brew reinstall -fs php71-mcrypt