Deprecation of the formats DATE_ISO8601 and DATE_RFC7231

outdated format, standard has been changed while ago

What do you mean? Do you mean that the constant was legal at some point?
Was there ever a version of ISO8601 allowing the mixing of "extended
format" and "basic format" ? The earliest version I've read, iso8604:2004,
does not allow it (more on that here:
https://externals.io/email/114883/source - can't find the proper link right
now, am on phone). Haven't read the older 1988/1999/2000 versions, and it's
possible one of those allowed it, but I doubt it. The only thing I can say
for sure is that it's been illegal since the 2004 version

How common is it in the wild? I am pretty sure I've used it myself
without really thinking many times, which I suspect is common. Knowing how
much code would be impacted would determine how we'd go about deprecating
it and on what timeline.

I'm not sure how would be possible to find out. I suppose most of the
frameworks (and thus popular PHP libraries) avoid this constant as showed
in Drupal and Carbon. The scanning of popular libraries will not show the
scale of the problem (however I will do it tomorrow) Helpful fact is that
those constants are just constants so it's easily replaceable with
strings 'Y-m-d\TH:i:sO'
and 'D, d M Y H:i:s \G\M\T'.

DATE_ISO8601 doesn't have to be removed anytime soon, but no new code
should be written using that constant, thus an E_DEPRECATED is
warranted.

Is anyone really arguing against that statement?

In my opinion, deprecating this does not do anything besides annoying
users.

In my opinion, since it isn't, and likely never was, a legal ISO8601
string, it's a no-brainer that it should be deprecated. (it's at least
been illegal since iso8601:2004 released in 2004)

I would like to propose the deprecation of the constants DATE_ISO8601,
DateTimeInterface::ISO8601 and DateTimeInterface::RFC7231, DATE_RFC7231.

…

In my opinion the question is not whether this constant should be
deprecated, but when. I know that the problem was discussed in the past,
but I hope enough time has passed already to touch this topic again
although of the BC nature. [6] https://externals.io/message/113657#114885
[7]

In my opinion, deprecating this does not do anything besides annoying
users.

https://www.reddit.com/r/PHP/comments/hnd438/why_isnt_date_iso8601_deprecated/
Arguments for deprecating DATE_RFC7231:

• error prone nature, the format never really supported timezone. I'm not
  sure how it appeared in the code, but tests clearly lack cases for this
  format. I'm not sure if it was missed on purpose, but with tests it would
  be obvious that the format shouldn't ever appear in the core. [8]
  https://www.php.net/manual/en/class.datetimeinterface.php [9]
  https://github.com/php/doc-en/pull/2296

This should instead be fixed of being deprecated. It is a recent
addition after all.

cheers,
Derick

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In my opinion, since it isn't, and likely never was, a legal ISO8601
string, it's a no-brainer that it should be deprecated. (it's at least
been illegal since iso8601:2004 released in 2004)

I thought the same when I started the discussion. It's not good promotion
for PHP, when every mention about using proper ISO8601 format starts with
warning (use DATE_ATOM, not ISO_8601!). It doesn't need to be removed
straight in PHP 9. We can wait for next major version.

I just don't understand why keeping invalid concept in the language is
important for such simple structure like constant. I understand it's BC,
but it cannot be keep forever.

I would write RFC anyway to check the reception, but I need Karma to do it.
Could I ask someone for it?