<?php
$thisWeek = new DateTime('@1408706174'); // get the day of the week, Monday = 0, Sunday = 6
$dayOfWeek = ((int)$thisWeek->format('N'))-1;
// subtract the day of the week to get Monday of this week
$thisWeek->modify("-$dayOfWeek days");
// get the timestamp
$monday = $thisWeek->format('U');
var_dump($monday);