[PHP-DEV] Add ISO 8601 date format constant with millisecond precision

Hi !

Sorry derick for the duplicated email but I was not registered.

This is my first contribution, so I would appreciate any help on the
proposal and on following the project's contribution process.

I opened issue #23491 for a possible addition to ext/date:

    https://github.com/php/php-src/issues/23491

I would appreciate feedback before preparing an implementation.

PHP currently provides DATE_RFC3339_EXTENDED, which includes millisecond
precision. For a UTC DateTime, it produces:

    1970-01-01T00:00:00.000+00:00

Some applications need the equivalent representation with the ISO 8601 UTC
designator Z:

    1970-01-01T00:00:00.000Z

This is also the format produced by JavaScript's
Date.prototype.toISOString():

    new Date(0).toISOString();
    // "1970-01-01T00:00:00.000Z"

Would a predefined format for this representation be appropriate for
ext/date?

One possible API would be:

    DATE_ISO8601_MILLISECONDS_UTC
    DateTimeInterface::ISO8601_MILLISECONDS_UTC

with the format string:

    "Y-m-d\\TH:i:s.v\\Z"

For example:

    $date = new DateTimeImmutable('@0');
    echo $date->setTimezone(new DateTimeZone('UTC'))
        ->format(DATE_ISO8601_MILLISECONDS_UTC);

    // 1970-01-01T00:00:00.000Z

The name and API shape are open for discussion. Another option would be an
offset-preserving format:

    DATE_ISO8601_MILLISECONDS
    DateTimeInterface::ISO8601_MILLISECONDS

using:

    "Y-m-d\\TH:i:s.vP"

I understand that date format constants only define formatting; they do not
change the timezone of the DateTime object.
A format containing a literal Z would therefore require the caller to
normalize the object to UTC first.

For context, issue #14593 discusses the interpretation of the Z suffix when
parsing DateTime values:

    https://github.com/php/php-src/issues/14593

That issue concerns parsing and timezone representation, whereas this
proposal concerns predefined formatting constants.

I would appreciate feedback on:

1. Whether a millisecond-precision format is useful in addition to
   DATE_RFC3339_EXTENDED.
2. Whether an offset-preserving format or a UTC-suffix format is preferable.
3. Whether the proposed names follow the preferred ext/date convention.
4. Whether an RFC is required before implementation.

Thank you,

Théo Attali