PHP strtotime +1 week error - php

Question

I have a problem with strtotime and the following code (I want all calendar weaks in the given time range)
$all_dates = array();
$tempdate = '2014-01-01';
$enddate = '2015-03-01';
while (strtotime($tempdate) <= strtotime($enddate)) {
$all_dates[] = date('Y-W', strtotime($tempdate));
$tempdate = date('Y-m-d', strtotime($tempdate . '+1 week'));
}
var_dump($all_dates);
the var_dump produces the following output
array (
0 => '2014-01',
1 => '2014-02',
2 => '2014-03',
3 => '2014-04',
4 => '2014-05',
5 => '2014-06',
6 => '2014-07',
7 => '2014-08',
8 => '2014-09',
9 => '2014-10',
10 => '2014-11',
11 => '2014-12',
12 => '2014-13',
13 => '2014-14',
14 => '2014-15',
15 => '2014-16',
16 => '2014-17',
17 => '2014-18',
18 => '2014-19',
19 => '2014-20',
20 => '2014-21',
21 => '2014-22',
22 => '2014-23',
23 => '2014-24',
24 => '2014-25',
25 => '2014-26',
26 => '2014-27',
27 => '2014-28',
28 => '2014-29',
29 => '2014-30',
30 => '2014-31',
31 => '2014-32',
32 => '2014-33',
33 => '2014-34',
34 => '2014-35',
35 => '2014-36',
36 => '2014-37',
37 => '2014-38',
38 => '2014-39',
39 => '2014-40',
40 => '2014-41',
41 => '2014-42',
42 => '2014-43',
43 => '2014-44',
44 => '2014-45',
45 => '2014-46',
46 => '2014-47',
47 => '2014-48',
48 => '2014-49',
49 => '2014-50',
50 => '2014-51',
51 => '2014-52',
52 => '2014-01',
53 => '2015-02',
54 => '2015-03',
55 => '2015-04',
56 => '2015-05',
57 => '2015-06',
58 => '2015-07',
59 => '2015-08',
60 => '2015-09',
)
The Problem is entry number 52 with '2014-01', it should be '2015-01'. With the timerange 2015-01-01 till 2016-03-01 the result is ok. Is this a bug in strtotime function?
I'm using PHP version 5.5.10
Thanks for your help.

Answer 1

Change this line and use the o iso year:
$all_dates[] = date('o-W', strtotime($tempdate));
// ^

Answer 2

This has to do with ISO week numbers. Also, strtotime() is a bad way to do date math and PHP recommends against it (thanks to leap years and daylight savings). Use the DateTime() classes to do this with accuracy:
$startdate = new DateTime('2014-01-01');
$enddate = new DateTime('2015-03-01');
$interval = new DateInterval('P7D');
$period = new DatePeriod($startdate , $interval, $enddate);
foreach($period as $date) {
$all_dates[] = $date->format('o-W'); // o modifier for ISO year
}
var_dump($all_dates);
Demo