CydeSwype

// add the repo that has all the PHP goodness
brew tap Homebrew/homebrew-php

// install the ssh2 library
brew install php71-ssh2

// verify you've got ssh2 loaded - should return something like:
// Additional .ini files parsed => /usr/local/etc/php/7.1/conf.d/ext-ssh2.ini
// Registered PHP Streams => https, ftps, compress.zlib, compress.bzip2, php, file, glob, data, http, ftp, phar, zip, ssh2.shell, ssh2.exec, ssh2.tunnel, ssh2.scp, ssh2.sftp
// ssh2