http://en.wikipedia.org/wiki/Iso8601#Durations
This decimal fraction may be specified with either a comma or a full stop, as in 
"P0,5Y" or "P0.5Y".
Remember to accept both comma and a full stop.

http://joshp.me/dateinterval-milliseconds/
Quick class to get around it, while the bug still exists...
class DateIntervalFractions extends DateInterval {
	public $milliseconds;
	public function __construct($interval_spec) {
		$this->milliseconds = 0;
		$matches = array();
		preg_match_all("#([0-9]*[.,]?[0-9]*)
[S]#",$interval_spec,$matches);
		foreach ($matches[0] as $result)
			$original = $result;
			list($seconds,$milliseconds) = 
explode(".",substr($result,0,-1));
			$this->milliseconds = $milliseconds / 
pow(10,strlen($milliseconds) - 3);
                        // Replace the milliseconds back to seconds,
                        // and let the original constructor do the rest.
			$interval_spec = str_replace($original,$seconds . 
"S",$interval_spec);
		parent::__construct($interval_spec);

I'm surprised this bug has persisted since 2011-01-24.  This seems to have been implemented incorrectly from the beginning.  ISO 8601 has mentioned support for fractional parts since at least 2004.
That is, it supports a decimal fraction to the smallest time value in a representation.
For example, the following representations are valid ISO 8601 intervals, but will cause the DateInterval constructor to throw exceptions:
P0.5Y
P2.3D
P1DT1.5H
PT585.829S
I appreciate the attempted workaround by "jdp2234 at hotmail dot com", but it appears to only support milliseconds.  It doesn't seem to address fractions in other parts of interval representations.

This appears to still be an issue with PHP 7.2, despite them adding a microseconds field to DateInterval.  My current workaround is:
        $fractional = new DateInterval('PT0S');
        $fractional->f = ".{$microseconds}";
        $fractional->invert = $seconds < 0;
        return (new DateTime("@{$seconds}"))->add($fractional);

This may be considered more of a feature request than a bug, however, I'm assigning to Derrick and pinging them here.
When I looked at "Appendix A. ISO 8601 Collected ABNF" of RFC3339, I only see the definition of Seconds Duration as DIGIT, not fraction:
Durations:
   dur-second        = 1*DIGIT "S"
   dur-minute        = 1*DIGIT "M" [dur-second]
   dur-hour          = 1*DIGIT "H" [dur-minute]
   dur-time          = "T" (dur-hour / dur-minute / dur-second)
   dur-day           = 1*DIGIT "D"
   dur-week          = 1*DIGIT "W"
   dur-month         = 1*DIGIT "M" [dur-day]
   dur-year          = 1*DIGIT "Y" [dur-month]
   dur-date          = (dur-day / dur-month / dur-year) [dur-time]
Additionally, section 5.3 of the RFC may seem to indicate that this not required by the specification:
5.3. Rarely Used Options
   A format which includes rarely used options is likely to cause
   interoperability problems.  This is because rarely used options are
   less likely to be used in alpha or beta testing, so bugs in parsing
   are less likely to be discovered.  Rarely used options should be made
   mandatory or omitted for the sake of interoperability whenever
   possible.
   The format defined below includes only one rarely used option:
   fractions of a second.  It is expected that this will be used only by
   applications which require strict ordering of date/time stamps or
   which have an unusual precision requirement.
However, I'll leave this to Derick to decide. It does seem to be a highly sought after feature and it would make sense to add it in the interval spec.
Derick, any thoughts on this?

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
Closing.
This feature won't be implemented in php at the current time.

According to the RFC cited as the reason for closing this report:
"This information is based on the 1988 version of ISO 8601.  There may
   be some changes in the 2000 revision."
However, when discussing the basis for the accepted formats for DateInterval, the PHP documentation (https://www.php.net/manual/en/dateinterval.construct.php) links not to the cited RFC but to the Wikipedia page for ISO 8601, which reflects more recent versions of the ISO that were written after 1988. If PHP devs insist this is not a bug, then the documentation should be updated to point to the RFC, and not to a page that discusses features of the ISO that PHP has not implemented.

For reference, here is a more recent version of ISO 8601 that I've found (from 2000). http://xml.coverpages.org/ISO-FDIS-8601.pdf
The relevant section is 5.5.3. It states:
"If necessary for a particular application the lowest order component may have a decimal fraction. The decimal
fraction shall be divided from the integer part by the decimal sign specified in ISO 31-0: i.e. the comma [,] or
full stop [.]. Of these, the comma is the preferred sign. The decimal fraction shall at least have one digit. If the
magnitude of the number is less than unity, the decimal sign shall be preceded by a zero (see ISO 31-0)."

Actually, as stated before, the standard allows fractions in the lowest part of the interval. Today I stumbled upon this when implementing an API to Polar watches. They actually send duration strings like "PT13724.627S", which seems to be ok by the RFC, but results in a "unknown or bad format" error in PHP.
I really think this is a bug and not a feature request.

As this is not considered a bug, at least the documentation should get updated. The documentation states, that the format is based on ISO 8601 including a link to Wikipedia, where it is stated, that fractions are allowed. Make it clear that this followes the rules of RFC 3339 or, at least, add a note to the documentation that fractions are not allowed.
Independet of the formal things, in my opinion this should be considered a bug. Citing RFC 3339 is, in my opinion, not expedient because the RFC itself says "when representing and using date and time in Internet protocols". Didn't know that PHP is an "internet protocol", and even PHP itself, despite often used for websites, describes itself as "PHP is a widely-used open source general-purpose scripting language". If it is a general-purpise scripting language, it should also allow fractions as mentioned in the ISO norm.