DUnit (dee-unit) makes your life easier by allowing you to run your unit tests on different versions of PHP or HHVM. Different versions are possible by using Docker containers. Thanks to @danbruce each of the Docker containers are only, ~35 MB in size. [...] PHP7 isn’t that far away. In fact, it is scheduled for release in ~8 months in November. Now is the perfect time to start testing your code against PHP7 nightly.
He starts with a few reasons you might want to test your code and things you can do to start "thinking forward" to when it is released. He then shows you how to install DUnit (more detail here) and use it to test on both PHP 7 and HHVM builds.