- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$monday = date("Y-m-d 00:00:00");
echo date("Y-m-d H:i:s",strtotime(date("Y-m-d 00:00:00",strtotime($monday))));
echo date("Y-m-d H:i:s",strtotime(date("Y-m-d 23:59:59",strtotime($monday))));