what you need is to tell PHP to be smarter
$the_date = strtotime('31.01.2011');
echo date('r', strtotime('last day of next month', $the_date));
$the_date = strtotime('31.03.2011');
echo date('r', strtotime('last day of next month', $the_date));
assuming you are only interesting on the last day of next month
reference – http://www.php.net/manual/en/datetime.formats.relative.php