Hi!
In PHP, you should use Vincent's approach (get timestamp and add a number of seconds to it, then format with date() function).
You can also feed mktime() with 'invalid' date arguments (such as $day + number_of_days_to_add) and then use date() to format the timestamp.
$tomorrow = mktime (0,0,0,date("m") ,date("d")+1,date("Y"));
$lastmonth = mktime (0,0,0,date("m")-1,date("d"), date("Y"));
$nextyear = mktime (0,0,0,date("m"), date("d"), date("Y")+1);
// this is example from PHP docs
If you need to use date logic in SQL, then use this (MySQL):
SELECT * from YourTable WHERE DATE_ADD(YourDaateColumn, INTERVAL X DAY) >= CURDATE();
Best regards,
Stas