So you found this post because you just updated or installed PHPUnit, you're running PHP 5.6, and you get this error:
"This version of PHPUnit is supported on PHP 7.0 and PHP 7.1.
You are using PHP 5.6.30 (/usr/local/Cellar/php56/5.6.30_6/bin/php)."
Maybe, like me, you're cranky about virtualization and you're using a PHP switcher to switch between PHP 5.6 and 7.1.
brew install [email protected]