Hi,
Simpler is to use UNIX timestamps.
But if you wish to use humanreadable format a la "12/11/1003" then you should use mktime function to get current timestamp, then add 60x60x24 secs and format with date back to correct format.
$today = "12/11/1003";
list($m,$d,$y)=explode($date);
$tomorrow_timestamp = mktime( 0,0,0,$m,$d,$y) + 60x60x24;
$tomorrow = date("m/d/Y", $tomorrow_timestamp);
print $tomorrow;