This really depends on how your date is formatted from the mySQL database.
However, let's just say that you can get the month, the day, and the year from the dB.
The following code will add 30 days to whatever date you have:
$thirtydays= mktime(0, 0, 0, $month, $day+30, $year);
if you want to just add one month, this will do it:
$onemonth= mktime(0, 0, 0, $month+1, $day, $year);
-Matt