date_default_timezone_set('Europe/Istanbul');

$first_day_of_the_week = 'Monday';
$start_of_the_week     = strtotime("Last $first_day_of_the_week");
if (strtolower(date('l')) === strtolower($first_day_of_the_week)){
    $start_of_the_week = strtotime('today');
}
$end_of_the_week = $start_of_the_week + (60 * 60 * 24 * 7) - 1;
$date_format =  'd-M-Y l H:i:s';

echo date($date_format, $start_of_the_week);
echo "<br>";
echo date($date_format, $end_of_the_week);