$ brew install php@5.6
$ brew install php@7.0
$ brew install php@7.1
$ brew install php@7.2


/usr/local/etc/php/5.6/php.ini
/usr/local/etc/php/7.0/php.ini
/usr/local/etc/php/7.1/php.ini
/usr/local/etc/php/7.2/php.ini


brew unlink php@7.2 && brew link --force --overwrite php@5.6

php -v

PHP 5.6.35 (cli) (built: Mar 31 2018 20:21:31)