PHP：从1969年开始获取周数

Question

I need to get the actual week number starting by 1969 year like this:

$dateTime = new DateTime;
$now = '2017-01-01';

// 
// $week = date('W', strtotime($now . ' +2 days'));
// I will get the week number between 01-53 and this is not what I want.
// I need to set the starting year and get the week number since 1969 to get the expected results.
// How to get the value 2506 from this $week variable?
//

$dateTime->setISODate('1969', 2506, -1);
$start_by = $dateTime->format('Y-m-d');

// output: 2016-12-31

As you can see, the '2016-12-31' is the datetime of my week start.

Thank you in advance!

Answer 1

Seems simple,

 <?php 
    $date1 = new DateTime();
    $date2 = new DateTime("1969-01-21");
    $interval = $date1->diff($date2);

    echo ceil($interval->days / 7);
 ?>