Use strtotime()
function like this:
strtotime("next monday");
strtotime("this sunday");
strtotime("last sunday");
In your case:
strtotime("this monday");
As a result you have timestamp.
The second way is like this:
$day = date('w');
$week_start = date('m-d-Y', strtotime('-'.$day.' days'));
$week_end = date('m-d-Y', strtotime('+'.(6-$day).' days'));