php time zone conversion conversion function

/*
 *  Time zone conversion 
 */

function toTimeZone($src, $from_tz = 'America/Denver', $to_tz = 'Asia/Shanghai', $fm = 'Y-m-d H:i:s') {
    $datetime = new DateTime($src, new DateTimeZone($from_tz));
    $datetime->setTimezone(new DateTimeZone($to_tz));
    return $datetime->format($fm);
}