Explain the concept of immutability

[samdark]

See yiisoft/factory#60

[mikehaertl]

Apart from the concept this should also include a clear guideline where to use it and where not. Also a couple of real world examples and use cases from inside the framework would surely help.

I remember seeing some places where it's used in yii3 - and others where it's not used but couldn't figure any difference in the context (can try to come up with examples). Is it possible that the new packages are not coherent in their use of immutability yet?

[samdark]

Is it possible that the new packages are not coherent in their use of immutability yet?

Yes.