Description:
------------
Currently creating a new DateTime(Immutable) Object using a timestamp does set the timezone as +00:00 (Type 1).

Expected outcome would be to have the timestamp be set to UTC (type 3).

Why? 

According to https://en.wikipedia.org/wiki/Unix_time the unix timestamp is always thought to be a representation of a DateTime in UTC. Having the timezone set to a simple offset does not reflect that. Even though the Unix timestamp has its own way of handling leap seconds and even though there are thoughts on coupling the unix timestamp to a (not yet generally used) "International time" (which would be UTC without leap seconds) the timestamp is generally considered to be a UTC representation, not just of offset +00:00.

See :

* https://en.wikipedia.org/wiki/Unix_time
* https://3v4l.org/f7bBe

Test script:
---------------
<?php

$f = new DateTimeImmutable('@1234567');
var_dump($f->getTimezone());


Expected result:
----------------
object(DateTimeZone)#2 (2) {
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(3) "UTC"
}

Actual result:
--------------
object(DateTimeZone)#2 (2) {
  ["timezone_type"]=>
  int(1)
  ["timezone"]=>
  string(6) "+00:00"
}