Install PHPCS with MAMP Pro
There are different ways to install PHPCS, the method below uses PEAR which installs it within your PHP installation.
First, locate the active MAMP Pro PHP installation.
$ which php
It will return something similar to /Applications/MAMP/bin/php/php7.2.1/bin/php. Go to the bin directory.
$ pear install PHP_CodeSniffer
That will install PHPCS.
As I bonus, configure it to work with WPCS (which has been already downloaded)
$ phpcs --config-set installed_paths ~/.scripts/wpcs
Notes:
When MAMP updates are installed, they generally include updated versions of PHP. So make a note that when you update MAMP (or if you toggle between PHP versions) you will need to reinstall PHPCS.