YYYY-MM-DD hh:mm:ss to unixtime

$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 ...;
 
*/