$modified_flag : boolean
This abstract class allows to know whether an object has been modified or not since it was created or read from persistent storage
Each time a property is modified, the descendant class must call the \Phool\setModified() method.
When it is saved, \Phool\clearModified() must be called
The current state can be retrieved via the \Phool\modified() method.
When the instance is created, the state is set to 'not-modified'