$birthday = '1980-06-01 00:00:00';
$birthdayunx = mktime(0, 0, 0, substr($birthday, 5, 2), substr($birthday, 8, 2), substr($birthday, 0, 4));
// the next day
$nextdayunx = mktime(0, 0, 0, substr($birthday, 5, 2), (substr($birthday, 8, 2) + 1), substr($birthday, 0, 4));
$nextdaystr = strftime("Y-m-d H:i:s", $nextdayunx);
/*
FROM:
int mktime ([ int $hour = date("H") [, int $minute = date("i") [, int $second = date("s") [, int $month = date("n") [, int $day = date("j") [, int $year = date("Y") [, int $is_dst = -1 ]]]]]]] )
If you can, use mysql date functions:
SELECT duedate, DATE_ADD(FROM_TIMESTAMP(duedate), INTERVAL 1 DAY) AS nextday FROM ...;
*/