This 4+ years old bug is still present in todays systems:
Ubuntu 10.04 / PHP 5.3.2-1ubuntu4.27
Ubuntu 12.04 / PHP 5.3.10-1ubuntu3.14
Ubuntu 14.04 / PHP 5.5.9-1ubuntu4.4

This bug is present in PHP 5.5 for Windows.

Confirmed on Mac. PHP version 5.5.26

Still broken in PHP7:

https://3v4l.org/aKNVI

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

DateTime::ISO8601 is defined to "Y-m-d\TH:i:sO"[1], so obviously
it doesn't accept microsecond fractions. If you expect these to be
given, use another format specifier, see <https://3v4l.org/ADn0l>.

If you want to accept different ISO-8601 date/time formats,
don't use date_create_from_format(), but rather date_create(),
see <https://3v4l.org/c879B>.

[1] <http://php.net/manual/en/class.datetime.php>

I would like to refer to this Wikipedia article:
https://en.wikipedia.org/wiki/ISO_8601

It states:
A decimal mark, either a comma or a dot (without any preference as stated in resolution 10 of the 22nd General Conference CGPM in 2003,[14] but with a preference for a comma according to ISO 8601:2004)[15] is used as a separator between the time element and its fraction.

@CMB you're wrong imo. in the sample code provided, PHP is told to parse a valid ISO8601 date, and PHP fails to do so. that's a bug in my eyes. please explain how this is not a bug, is PHP not supposed to parse a ISO8601 date when told to do so?

> PHP is told to parse a valid ISO8601 date

No.  It is told to use the DateTime::ISO8601 format,
and the docs[1] state:

| This format is not compatible with ISO-8601, but is left this
| way for backward compatibility reasons. Use DateTime::ATOM or
| DATE_ATOM for compatibility with ISO-8601 instead.

If you think DateTime::ISO8601 should be changed for PHP 8, please
pursue the RFC process[2].

[1] <https://www.php.net/manual/en/class.datetimeinterface.php#datetime.constants.iso8601>
[2] <https://wiki.php.net/rfc/howto>

For anyone reading this: 

DateTime::ATOM / DATE_ATOM suffers from exactly the same problem, and there's currently (as of PHP 8.0) no easy way to correctly handle milliseconds with built-in PHP functions.

If you want to read the ISO 8601 format as produced by Javascript's Date.toJSON() in PHP, you have to rely on third party tools. 

I recommend this one: https://carbon.nesbot.com/

Here a value from [the documentation](https://www.php.net/manual/en/datetime.format.php) is used:

    $value = '2004-02-12T15:19:21+00:00';
    $dt = new DateTime($value);
    $formatted = $dt->format('c');
    $date = \DateTime::createFromFormat('c', $formatted);
    $errors = \DateTime::getLastErrors();

Now, this produces the following errors:
- The format separator does not match
- Trailing data

This is funny, because value equals the formatted value ????‍♀️.
$formatted === $value // TRUE

Why can't we parse the value PHP itself outputs?

Sorry, this seems to be known issue.

Using DATE_ATOM / DateTimeInterface::ATOM will work for ISO 8601 string NOT containing decimal parts.

It does NOT work for strings with decimal parts, though.