I want result in hours between two datetime

Question

Here, I want to get hours between two dates. If I subtract time between two dates, I want 23 hours in a result.

My field's datatype is as datetime.

My code is like,

$usr_check_login = $this -> applib->get_any_field('users_attendance',array('user_id'=>$this->session->userdata('user_id'),'clock_out'=>'0000-00-00 00:00:00'),'clock_in');



            $cur_time = date('d-m-Y H:i:s');
            $clk_time = date('d-m-Y H:i:s',strtotime($usr_check_login));


            $res = $cur_time - $clk_time;
            $hours = $res / ( 60 * 60 );
            echo $hours."<br>";
            echo $cur_time."</br>";
            echo $clk_time."</br>";

I am getting result like,

0.00027777777777778
23-01-2016 11:29:19
22-01-2016 11:02:39

Here I want result as 24 hours as the dates are changed.

What can be the solution?