- mktime: documentation ( source)
- date: documentation ( source)
<?php
$dayoftheweek = date('w'); //PHP : from 0 (sunday) to 6 (pour saturday)
if ($dayoftheweek == 0)
$dayoftheweek = 7;
$monday= mktime(0, 0, 0, date("m"), (date("d") - $dayoftheweek + 1), date("Y"));