Missing Week in PHP's DateTime->modify('next week') - php

Question

I think I am fully aware of ISO 8601 and that the first week of a year is the week that has a Monday in it. However I came across a strange behavior in PHP (5.6) DateTime Class.
Here is my code:
$start = new DateTime('2009-01-01 00:00');
$end = new DateTime();
$point = $start;
while($point <= $end){
echo $point->format('YW');
$point = $point->modify('next week');
}
This puts out correctly
200901
200902
200903
...
But if I pick as a start date something earlier in 2008 like $start = new DateTime('2008-01-01 00:00'); then I get a different result:
...
200852
200801 // <=== 2008??
200902
200903
...
Is this a PHP bug or am I'm missing something here?

Answer 1

Tinkered with this and finally figured it out
$start = new DateTime('2008-12-29 00:00');
$end = new DateTime('2009-01-7 00:00');
$point = $start;
while($point <= $end){
echo $point->format('YW') . "\t";
echo $point->format('m-d-Y') . "\n";
$point = $point->modify('next week');
}
So the first date here is 2008-12-29. Thus Y is correct. But 2008-12-29 is also week 1. So the W is also correct
https://3v4l.org/JZtqa

Answer 2

It's not a bug! Inspired by #Machavity and based on this this similar question I found a solution:
echo $point->format('oW');
instead of
echo $point->format('YW')
produces:
...
200852
200901
200902
...
no matter when the start date is. It's really a RTM case, as the PHP manual states:
o ==>
ISO-8601 year number. This has the same value as Y, except that if the
ISO week number (W) belongs to the previous or next year, that year is
used instead. (added in PHP 5.1.0)