/**
* 生成UUID
* @param string $prefix
* @return string
*/
function uuid($prefix = ''){
$char = strtoupper(md5(uniqid(mt_rand(),true)));
$uuid = substr($char,0,8).'-';
$uuid .= substr($char,8,4).'-';
$uuid .= substr($char,12,4).'-';
$uuid .= substr($char,16,4).'-';
$uuid .= substr($char,20,12);
return $prefix.$uuid;
}