Re: Questions regarding DateTimeImmutable

>> a) The DateTimeImmutable class extends the DateTime class. Why was >> this done this way? Both classes are incompatible (as in the >> inheritance violates LSP). E.g. if you hint against DateTime, then >> passing in DateTimeImmutable will not work (as the behavior is >> different). The same also applies the other way around (DateTime >> extending DateTimeImmutable). Imho a mutable and an immutable variant >> of a class can't reasonably extend from one another, so I'm not sure >> why this was done. > > It was mostly done so that methods and code accepting do not need > signature changes. In many cases library code would accept a DateTime, > call modify and return the changed object - not realizing that the > original object was changed as well causes issues with calling code. By > having DateTimeImmutable extend DateTime they can continue to pass them > in, without having to change library code at all. What if they return the same object knowing that modify actually modifies the object? I personally would have done that except in the lucky situation that I used it in the return statement: return $timestamp->modify('+30 days'); Then I'd luckily work with DateTimeImmutable. --- While I personally think DateTime should have been immutable from the beginning, I don't think it's in PHP's best interest to try to fix this particular problem by introducing DateTimeImmutable.