There is no date_add() PHP function. That's a MySQL function - see:
http://dev.mysql.com/doc/refman/5.0/en/date-and-time-functions.html#id3094648
In PHP you can do something like this:
$time = date('Y-m-d H:i:s', strtotime('+1 hour'));
Sometimes that doesn't work well on Windows platform. In that case, you can do this method which should work for Windows and Unix platforms:
$time = date('Y-m-d H:i:s', time() + 3600); // 3600 seconds = 1 hour
It's best to do it from MySQL if that's what you're trying to use:
SELECT DATE_ADD(NOW(), INTERVAL 1 HOUR) AS current_ts FROM table_name
hth.