日時のISO 8601形式の変更

$y = mb_substr(date("Ymd"), 0, 4, "utf8");
$m = mb_substr(date("Ymd"), 4, 2, "utf8");
$d = mb_substr(date("Ymd"), 6, 2, "utf8");
$dt = new DateTime("{$y}-{$m}-{$d}T00:00:00", new DateTimeZone("Asia/Tokyo"));
$date = $dt->format('c');