- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$get = '2017-07-16';
$date = new DateTime($get, new DateTimeZone('Europe/Helsinki'));
$date = $date->format('Y-m-d');
echo $date .' ';
$weekdate = date('Y-m-d', strtotime('last Monday', strtotime($date)));
$weekdate = strtotime($weekdate);
echo $weekdate . ' ';
echo date('Y-m-d', $weekdate);