<?php
date_default_timezone_set('PRC');
function udate($format='Y-m-d H:i:s.u', $utimestamp='') {
    empty($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'); // 12:33:03.599516
echo udate('Y-m-d H:i:s.u'); // 2016-10-16 12:33:03.599684
echo udate('Y-m-d H:i:s.u', 1234567890.654321); // 2009-02-14 07:31:30.654321