9 balls and scales

You have 9 balls, equally big, equally heavy - except for one, which is a little heavier. How would you identify the heavier ball if you could use a pair of balance scales only twice?

DEMO: http://dev.ibelik.com/9balls/

Screenshot:

Screenshot, scheme and dump located in the data directory.

Install:

1. composer install

2. bower install

3. vendor/bin/phinx init

4. edit phinx.yml config

5. vendor/bin/phinx migrate

Test:

php vendor/phpunit/phpunit/phpunit tests/BallsTest