Description:
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I am currently in the process of migrating my codebase to PHP 5.5

I do a lot of stuff with dates, so a big thing for me is to move everything to DateTimeImmutable.

What this generally implies for me is:

1. I accept DateTimeInterface everywhere.
2. I emit DateTimeImmutable everywhere.

Often times I will want to work internally with DateTimeImmutable. So I really need a way to 'cast' whatever I have to DateTimeImmutable.

Unfortunately we only have:

DateTimeImmutable::createFromMutable

This means that I'll generally always have to check if I got a DateTime or DateTimeInterface, and only call createFromMutable _if_ it's a DateTime object.

No huge deal, but it would be nicer if I can always call:

DateTimeMutable::createFromInterface(DateTimeInterface $dt).

But one step beyond that would be, and would be even more convenient, is if DateTimeInterface got a method :

getImmutable() 

In the case of a DateTime we would get a Immutable copy of it, in the case of DateTimeImmutable, it can just return itself.