What is the value being returned by wp_timezone()?

"Europe/Berlin"

wp_timezone()->getName() is what I logged and it was "Europe/Berlin"

<?php
$datetime = date_create("2020-07-13 10:38:15", timezone_open("Europe/Berlin")); date_timezone_set( $datetime, timezone_open( "UTC" ) ); date_format( $datetime, "Y-m-d H:i:s" ); var_dump($datetime);

Expected result:
----------------
object(DateTime)#5 (3) {
  ["date"]=>
  string(26) "2020-07-13 08:38:15.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(3) "UTC"
}
timezone now is UTC?

The result is the same with this command sequence.

Workstation (correct):

    $ php -r '$datetime = date_create("2020-07-13 10:38:15", timezone_open("Europe/Berlin")); date_timezone_set( $datetime, timezone_open( "UTC" ) ); date_format( $datetime, "Y-m-d H:i:s" ); var_dump($datetime);'
    Command line code:1:
    class DateTime#2 (3) {
      public $date =>
      string(26) "2020-07-13 08:38:15.000000"
      public $timezone_type =>
      int(3)
      public $timezone =>
      string(3) "UTC"
    }

Server (wrong):

    $ php -r '$datetime = date_create("2020-07-13 10:38:15", timezone_open("Europe/Berlin")); date_timezone_set( $datetime, timezone_open( "UTC" ) ); date_format( $datetime, "Y-m-d H:i:s" ); var_dump($datetime);'
    object(DateTime)#2 (3) {
      ["date"]=>
      string(26) "2020-07-13 10:38:15.000000"
      ["timezone_type"]=>
      int(3)
      ["timezone"]=>
      string(3) "UTC"
    }

Now this is very strange.  I get the correct result if I check out PHP 7.4.8 (same version as the REMI packages) from Git, compile it on CentOS 7, and run this php executable on the server. So in the same environment and while ignoring php.ini (with -n) I get different results for the same version.

# installed from remi repo:
# php-cli-7.4.8-2.el7.remi.x86_64
# php-7.4.8-2.el7.remi.x86_64

$ php --version
PHP 7.4.8 (cli) (built: Jul  9 2020 08:57:23) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies


$ php -n -r '$datetime = date_create("2020-07-13 10:38:15", new DateTimeZone("Europe/Berlin")); var_dump($datetime); $datetime->setTimezone( new DateTimeZone( "UTC" ) ); var_dump($datetime);'

object(DateTime)#2 (3) {
  ["date"]=>
  string(26) "2020-07-13 10:38:15.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(13) "Europe/Berlin"
}
object(DateTime)#2 (3) {
  ["date"]=>
  string(26) "2020-07-13 10:38:15.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(3) "UTC"
}


# self-compiled on centos 7 => correct result

$ sapi/cli/php --version
PHP 7.4.8 (cli) (built: Jul 25 2020 22:12:24) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies


$ sapi/cli/php -n -r '$datetime = date_create("2020-07-13 10:38:15", new DateTimeZone("Europe/Berlin")); var_dump($datetime); $datetime->setTimezone( new DateTimeZone( "UTC" ) ); var_dump($datetime);'

object(DateTime)#2 (3) {
  ["date"]=>
  string(26) "2020-07-13 10:38:15.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(13) "Europe/Berlin"
}
object(DateTime)#2 (3) {
  ["date"]=>
  string(26) "2020-07-13 08:38:15.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(3) "UTC"
}

I observed a similar issue with PHP 7.2.5 on SLES 12 SP 5: 

<?php

date_default_timezone_set('UTC');
echo date('Y-m-d: H:i:s e');
date_default_timezone_set('Europe/Berlin');
echo date('Y-m-d: H:i:s e');
date_default_timezone_set('Asia/Irkutsk');
echo date('Y-m-d: H:i:s e');

Returns: 
2020-12-04: 12:43:34 UTC
2020-12-04: 12:43:34 Europe/Berlin
2020-12-04: 12:43:34 Asia/Irkutsk

Changes in /etc/php/cli/php.ini date.timezone did not make difference. 
I have no direct access to the server, but could provide more information regarding the server environment.

From php 5.4+ it retrieves the time zone if set via date_default_timezone_get or from the php.ini date.timezone setting or set to UTC if absent date.timezone and date_default_timezone_get. From php 7 if date.timezone is not set it does not show warning. for more information read here https://www.php.net/manual/en/datetime.configuration.php#ini.date.timezone

https://3v4l.org/0enPY
I don't think it's a php bug but maybe it doesn't do that php expects and therefore it's an error even with date_default_timezone_get https://www.php.net/manual/en/function.date-default-timezone-get.php#113202

Note* WordPress set date_default_timezone_set("UTC"); https://github.com/WordPress/WordPress/blob/5.6-branch/wp-settings.php#L68

> So in the same environment and while ignoring php.ini (with -n)
> I get different results for the same version.

So this is either a bug in the CentOS supplied version, or might
be related to different timezonedb[1] versions.  Either way,
doesn't look like a PHP bug.

[1] <https://pecl.php.net/package/timezonedb>