Parse a UTC date, output local datetime

I have already read How to get local time in php? and similar questions, but here I want to do the contrary:

With a given datetime in UTC (e.g. 2021-03-31 23:45:00), how to output the date in local timezone?

$dt = new DateTime("2021-03-31 23:45:00");  // parse the UTC datetime
echo $dt->format('m/d/Y, H:i:s');           

In Europe/Paris timezone, it should output 2021-04-01 01:45:00, but here it sill outputs 2021-03-31 23:45:00. How to fix this?

I also tried with strtotime with a similar result; I also tried with:

date_default_timezone_set('UTC');
$dt = new DateTime("2021-03-31 23:46:14");
date_default_timezone_set('Europe/Paris');
echo $dt->format('m/d/Y, H:i:s');

without success.

Answer

You need to change the timezone of the date (using DateTime::setTimeZone()), not the default timezone:

date_default_timezone_set('UTC');
$dt = new DateTime("2021-03-31 23:46:14");
$dt->setTimeZone(new DateTimeZone("Europe/paris")); // change date timezone
echo $dt->format('m/d/Y, H:i:s');

Output:

04/01/2021, 01:46:14

Changing the default timezone affects the new DateTime(), not the format() result.