PHP - Add one week to a user defined date

You can try this

$start_date = "2015/03/02";  
$date = strtotime($start_date);
$date = strtotime("+7 day", $date);
echo date('Y/m/d', $date);

Object Oriented Style using DateTime classes:

$start_date = DateTime::createFromFormat('Y/m/d', $_POST['start_date']);

$one_week = DateInterval::createFromDateString('1 week');

$start_date->add($one_week);

$date_to_pay = $start_date->format('Y/m/d');

Or for those who like to have it all in one go:

$date_to_pay = DateTime::createFromFormat('Y/m/d',$_POST['start_date'])
                       ->add(DateInterval::createFromDateString('1 week'))
                       ->format('Y/m/d');

Please try the following:

date('d.m.Y', strtotime('+1 week', strtotime($start_date)));