- date_default_timezone_set: documentation ( source)
<?php
date_default_timezone_set("UTC");
echo (new DateTime("2016-06-19"))->modify("Monday this week")->format("c"), PHP_EOL;
echo (new DateTime("2016-06-19"))->format("W"), PHP_EOL, (new DateTime("2016-06-20"))->format("W"), PHP_EOL;