$adate = '2013-12-10'; //can use Ymd or mdY. Can't use dmY. Can use dash or slash to separate
$daystoadd = 7;
$newdate = date('Y-m-d',strtotime($adate) + (24*3600*$daystoadd)); //the date format is for output

$adate = '2013-12-10'; //can use Ymd or mdY. Can't use dmY. Can use dash or slash to separate $daystoadd = 7; $newdate = date('Y-m-d',strtotime($adate) + (24*3600*$daystoadd)); //the date format is for output