calculate hours between two times in php

  $difference=0;
                if($task[$i]['in_date_time'] && $task[$i]['out_date_time']){
                $expiry_time = new DateTime($task[$i]['in_date_time']);
                $current_date = new DateTime($task[$i]['out_date_time']);
                $diff = $expiry_time->diff($current_date);
                $difference= $diff->format('%H:%I'); 
                }            
                 echo $difference;