function udate($format = 'u', $utimestamp = null) {
if (is_null($utimestamp))
$utimestamp = microtime(true);
$timestamp = floor($utimestamp);
$milliseconds = round(($utimestamp - $timestamp) * 1000000);
return date(preg_replace('`(?<!\\\\)u`', $milliseconds, $format), $timestamp);
}
echo udate('Y-m-d H:i:s.u');