Be aware that setTime can cause a change in the timezone offset: https://3v4l.org/MqYN9
(The time 01:05:00 exists twice on this day in Europe/London due to DST change - once in +01:00 and then again at +00:00)
$tzUK = new \DateTimeZone("Europe/London");
$tzUtc = new \DateTimeZone("UTC");
$dt = \DateTimeImmutable::createFromFormat("!Y-m-d H:i:s", "2020-10-25 00:05:00", $tzUtc);
$dt = DateTime::createFromFormat('U', $dt->format('U'));
print $dt->format(\DateTime::RFC3339 ." e") ."\n";
$dt->setTimeZone($tzUK);
print $dt->format(\DateTime::RFC3339 ." e") ."\n";
$dt->setTime((int) $dt->format('H'), (int) $dt->format('i'), 0);
print $dt->format(\DateTime::RFC3339 ." e") ."\n";
Will output:
2020-10-25T00:05:00+00:00 +00:00
2020-10-25T01:05:00+01:00 Europe/London
2020-10-25T01:05:00+00:00 Europe/London
Verified on PHP 5.3 thru 8.0 (latest at time of posting)