- var_dump: documentation ( source)
<?php
$yearMonth = '2017-06';
$firstDayOfThisMonth = new \DateTime($yearMonth . '-01');
var_dump($firstDayOfThisMonth->format('W'));
$lastDayOfThisMonth = clone $firstDayOfThisMonth;
$lastDayOfThisMonth = $lastDayOfThisMonth->modify('last day of this month');
var_dump($lastDayOfThisMonth->format('W'));