$time = time() - strtotime($created_at); // $created_at is a string of time i.e. Fri Mar 18 17:24:35 PDT 2011

            $days = floor($time/24/60/60);

            if ($days > 0) {

                $days_out = "$days day";

                $days_out .= $days > 1 ? 's' : '' .", ";

            }

            $hours = floor(($time % (24*60*60))/60/60);

            $hours_out = "{$hours} hour";

            $hours_out .= $hours > 1 ? 's' : '' . " ago";

            echo "{$days_out}{$hours_out}";