在PHP中规范化"时间"的最佳方法是什么？

$time_str = "2022-01-01 12:00:00";
$date_time = new DateTime($time_str);

$date_time = new DateTime("2022-01-01 12:00:00");
$time_str = $date_time->format("Y-m-d H:i:s");

$date_time = new DateTime("2022-01-01 12:00:00");
$formatted_date_time = $date_time->format("Y-m-d H:i:s.u");

$date_time = new DateTimeImmutable("now");
$formatted_date_time = $date_time->format("Y-m-d H:i:s.u");

$timestamp = time();
$formatted_date_time = date("Y-m-d H:i:s.u", $timestamp);