Always run the correct version of phpunit for each project. Add this to ~/bin/phpunit and ensure ~/bin is at the beginning of your PATH

phpunit

#!/usr/bin/env bash
if [ -f vendor/bin/phpunit ]; then
    vendor/bin/phpunit $@
    exit $?
fi

if [ -f ~/.composer/vendor/bin/phpunit ]; then
    ~/.composer/vendor/bin/phpunit $@
    exit $?
fi

if [ -f /usr/local/bin/phpunit ]; then
    /usr/local/bin/phpunit $@
    exit $?
fi

echo "I'm out of ideas. Can't find phpunit anywhere.  Maybe you should 'composer global require phpunit/phpunit'"
exit 1