Implement DateComparableInterval

[nikic]

Following @derickr's suggestion in #4039, this changes DateTime::diff() to return a new DateComparableInterval class, which has well-defined comparison semantics.

TODO:

• More comparison tests (only checked the tricky P1M case for now)
• Naming? I've used DateComparableInterval here, Derick suggested DateAbsoluteInterval.

cc @frederikbosch @Antnee

[frederikbosch]

Wonderful @nikic, I did not expect this PR/feature so soon!

Regarding the name, a class DateAbsoluteInterval suggests more features than DateComparableInterval. The latter, as the name indicates, suggests comparison only. So the question is whether we want to open this class for more than only comparison?

If we would name it DateAbsoluteInterval, which features would users expect from it?

1. To be able to construct it. new DateAbsoluteInterval('PT1S')
2. To be able to compare the objects.
3. ....

So my suggestion would be to name it DateComparableInterval if the constructor is going to be private and usage is for comparison only, and name it DateAbsoluteInterval if the constructor is going to be public with the possibility of more features.

@nikic: why should we want to make the constructor private? I understand that the constructor should be limited if we want to make it public (probably (milli)seconds PT[0-9]*S only), otherwise we would lose the comparison feature.

[derickr]

I think we should work on an RFC for this first @nikic

[frederikbosch]

@nikic @derickr Would you agree on my two alternatives? Then I will create a RFC that includes the two options to pick from.

[nikic]

@derickr First need to decide what the RFC is supposed to propose ;)

@frederikbosch A possible feature for DateAbsoluteInterval would be to get the total number of seconds.

why should we want to make the constructor private? I understand that the constructor should be limited if we want to make it public (probably (milli)seconds PT[0-9]*S only), otherwise we would lose the comparison feature.

We could allow anything up to (and including) days. I'm not sure how useful this would really be.

[nikic]

@derickr Any further thoughts on this?

[derickr]

@nikic Pretty much what it says in the ticket for the RFC?

[iluuu1994]

@derickr Do you think this is still relevant? If not feel free to close this PR.

[derickr]

I don't know. This PR certainly no longer applies, and changing the return of DateTime::diff() is a BC break. I'm therefore closing this and if somebody is interested again we can revisit.