- 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 .'\n';
$weekdate = date($date, strtotime('last Monday'));
$weekdate = strtotime($weekdate);
echo $weekdate;