Description:
------------
When using a timezone abbreviation (e. g. CEST) instead of a geographical timezone name (e. g. Europe/Berlin) in the DateTimeZone constructor, the resulting time offset is that of the corresponding non-DST timezone, coming up an hour short.

https://3v4l.org/VTI3C

Test script:
---------------
$dt = new DateTime(
    '2017-05-16T10:11:32',
    new DateTimeZone('CEST')
);
var_dump($dt);
var_dump($dt->format('c'));

Expected result:
----------------
object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2017-05-16 10:11:32.000000"
  ["timezone_type"]=>
  int(2)
  ["timezone"]=>
  string(4) "CEST"
}
string(25) "2017-05-16T10:11:32+02:00"

Actual result:
--------------
object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2017-05-16 10:11:32.000000"
  ["timezone_type"]=>
  int(2)
  ["timezone"]=>
  string(4) "CEST"
}
string(25) "2017-05-16T10:11:32+01:00"