- var_dump: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
var_dump(strtotime('first day'));
var_dump(date('Y-m-d H:i:s', strtotime('first day')));
var_dump(strtotime('first day 00:00:00'));
var_dump(date('Y-m-d H:i:s', strtotime('first day 00:00:00')));