Another way, I think the better way, would be to use:
$today=date('m/d/Y');
$week=date('m/d/Y',(time()+604800));
604800 is how many seconds are in a week.
time() returns the current time measured in the number of seconds since the Unix Epoch (January 1 1970 00:00:00 GMT).
I tested this for you:
<?
echo date('m/d/Y');
echo '<BR>';
$week=date('m/d/Y',(time()+604800));
echo $week;
?>
Which output:
03/14/2003
03/21/2003
Should work best for you.