Gives first day of the first week of the current month

firstDayofFirstWeekinCurrentMonth.php

<?php
/**
 * Created by PhpStorm.
 * User: Rudde
 * Date: 01.04.2016
 * Time: 00.25
 */

function firstDayofFirstWeekinCurrentMonth($date = NULL) {
    $date = is_null ($date) ? new DateTime('first day of this month') : $date->modify('first day of this month');
    $date->modify(('Sunday' == $date->format('l')) ? 'Monday last week' : 'Monday this week');

    return $date;
}