"String in a format accepted by strtotime()" is not 100% truth - you cannot pass timezone info in the string used as DateTime constructor, while you can do it with strtotime(). It may be a problem if you would like to create a date from GMT time and then display it in your local timezone, for example:
<?php
$timeZone = 'Europe/Warsaw'; date_default_timezone_set($timeZone);
$dateSrc = '2007-04-19 12:50 GMT';
$dateTime = new DateTime($dateSrc);
echo 'date(): '.date('H:i:s', strtotime($dateSrc));
echo 'DateTime::format(): '.$dateTime->format('H:i:s');
?>
[red. your claim that "is not 100% truth" is incorrect, you're seeing desired behavior here. The timezone passed as 2nd argument is used as a default fall back, in case the parsed string doesn't provide TZ information.]
So if you want to convert date between different timezones, you have to create two DateTimeZone objects - one for the input and one for output, like this:
<?php
$timeZone = 'Europe/Warsaw'; $dateSrc = '2007-04-19 12:50';
$dateTime = new DateTime($dateSrc, new DateTimeZone('GMT'));
$dateTime->setTimeZone(new DateTimeZone($timeZone));
echo 'DateTime::format(): '.$dateTime->format('H:i:s');
?>
I'm not sure if this is a bug or desired behaviour.
[red. you don't have to do create two DateTimeZone objects, this works too:
<?php
$timeZone = 'Europe/Warsaw'; $dateSrc = '2007-04-19 12:50 GMT';
$dateTime = new DateTime($dateSrc);
$dateTime->setTimeZone(new DateTimeZone($timeZone));
echo 'DateTime::format(): '.$dateTime->format('H:i:s');
?>
]