php 获得 精确 时间 有意思～！！～～！

/**
*
* @param <type> $format
* @param <type> $utimestamp
* @return <type>
* @example
*
* echo udate('H:i:s.u'); 19:31:28.7812
*
* echo udate('H:i:s.uuuu'); 19:31:28.78127781277812778127
*
* echo udate('Y-M-D H:i:s.u'); 2010-Apr-Thu 19:40:49.484376
*
* echo udate('Y-m-d H:i:s.u'); 2010-04-22 20:20:32.453127
*
* echo date("Ymd").udate('Hisu'); 20100422201737484376
*
* echo date("Ymd").udate('Hisuu'); 20100422201926156251156251
*
* @author wulei
* @date
*
*/
function udate($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);
}