No default values for created and changed, this has to be changed in all entities.
PHP has the static::class constant attached to each class, but no short name. I fiddled around with some functions (see trait in commit) but in the end another constant seems to be the best solution. Added the constant to the PHPClass template for NetBeans, so it's auto-generated.
Removing again, not as flexible as the originals. Will cause trouble with the other devs, as they don't know how to handle it, but they'll have to learn.
Get rid of the database base class. This makes inheritance chains shorter and allows us to use the database in static context. It's not a singleton and has not global state, nor any dependencies. That's what we need.
Continued with revision system, various fixes related to previous commits.