you can use datediff in your SQL query, like:
SELECT `id` , `opened` , `closed` , datediff( `opened` , `closed` ) AS days
FROM `tickets`
It gives you in seconds:
SELECT `opened`, `closed`, (`closed`-`opened`) AS seconds FROM `tickets` WHERE `closed`!='0000-00-00 00:00:00'
You can read about date and time function in the mysql manual:
Here