Had to install Yarn (brew install yarn). Already had composer installed. Needed to upgrade my local version of php:
https://coolestguidesontheplanet.com/upgrade-php-on-osx/
curl -s http://php-osx.liip.ch/install.sh | bash -s 7.0
Then point to it for php-cli:
Add export PATH=/usr/local/php5/bin:$PATH to .bashrc, then source ~/.bashrc, which is the file named .bashrc in my home directory.
Confirm with php --version.