//Object Oriented Method
$datetime = new DateTime('2010-12-30 23:21:46');
echo $datetime->format(DateTime::ATOM); // Updated ISO8601
//Procedural Method
echo date(DATE_ISO8601, strtotime('2010-12-30 23:21:46'));
(new DateTime('2022-05-01T16:00:00-03:00'))->format('Y-m-dTH:i:sp');
$timestamp = '1268932078';
$iso8601 = date('c', $timestamp);