php生成唯一图片uuid

function uuid() {
        $charid = md5(uniqid(mt_rand(), true));
        $hyphen = chr(45);// "-"
        $uuid = substr($charid, 0, 8).$hyphen
               .substr($charid, 8, 4).$hyphen
               .substr($charid,12, 4).$hyphen
               .substr($charid,16, 4).$hyphen
               .substr($charid,20,12);
        return $uuid;
}