About compatibility.
Test suite designed for PHPUnit 5 won't run on PHPUnit 6
But, thanks to "forward compatibility",
test suite designed for PHPUnit 6 will run on
PHPUnit 4.8.35 (EPEL-7, testing)
PHPUnit 5.7.* (Fedora)
Minimal adaptation can be enough (simple case)
Examples:
https://github.com/php-fedora/autoloader/commit/b548f49c56146f486e9fac862...
https://github.com/glpi-project/glpi/commit/1df387eb037214d9c0a419fbb013b...
Remi.