PHP DateTime microseconds always returns 0

<?php
$dt = new DateTime();
echo $dt->format("Y-m-dTH:i:s.u") . "n";
?>

[root@www1 ~]$ php date_test.php
2008-10-03T20:31:26.000000
[root@www1 ~]$ php date_test.php
2008-10-03T20:31:27.000000
[root@www1 ~]$ php date_test.php
2008-10-03T20:31:27.000000
[root@www1 ~]$ php date_test.php
2008-10-03T20:31:28.000000

function udate($format, $utimestamp = null)
{
    if (is_null($utimestamp))
        $utimestamp = microtime(true);

    $timestamp = floor($utimestamp);
    $milliseconds = round(($utimestamp - $timestamp) * 1000000);

    return date(preg_replace('`(?<!\\)u`', $milliseconds, $format), $timestamp);
}

echo udate('H:i:s.u'); // 19:40:56.78128

function getTimestamp()
{
        return date("Y-m-dTH:i:s") . substr((string)microtime(), 1, 8);
}

class ExtendedDateTime extends DateTime {
    /**
     * Returns new DateTime object.  Adds microtime for "now" dates
     * @param string $sTime
     * @param DateTimeZone $oTimeZone
     */
    public function __construct($sTime = 'now', DateTimeZone $oTimeZone = NULL) {
        // check that constructor is called as current date/time
        if (strtotime($sTime) == time()) {
            $aMicrotime = explode(' ', microtime());
            $sTime = date('Y-m-d H:i:s.' . $aMicrotime[0] * 1000000, $aMicrotime[1]);
        }

        // DateTime throws an Exception with a null TimeZone
        if ($oTimeZone instanceof DateTimeZone) {
            parent::__construct($sTime, $oTimeZone);
        } else {
            parent::__construct($sTime);
        }
    }
}

$oDate = new ExtendedDateTime();
echo $oDate->format('Y-m-d G:i:s.u');

2010-12-01 18:12:10.146625

date_create_from_format('U.u', microtime(true))->format('Y-m-d H:i:s.u');