To find Monday of next week, we change 'last' to 'next'.
To determine the distance of current day with the first day of week, we use function:
<?php
echo floor((strtotime ("now")- strtotime("last Monday"))/86400);
?>
2. Determine the first day of month
<?php
$day = date('Y-m-d', strtotime(date('Y-m-01', strtotime("now"))));
return $day;
?>
Or:
<?php
$date = date("01/m/Y");
echo $date;
?>