How can i add hours for the query in PHP

I have already added minutes and seconds. But I don’t know how to add hours to this. Can any one help me. I have given the all files of ine.This is my index.php file .

<div id="response"> </div>

<script type="text/javascript">
  var obj =   setInterval(function(){
   var xmlhttp = new XMLHttpRequest();
   xmlhttp.open("GET" , "response.php" , false);
   xmlhttp.send(null);
   document.getElementById("response").innerHTML =xmlhttp.responseText; 


    },1000);
 </script>

This is my first.php file.

 <?php 
    
        include("databaseconn.php");
        session_start();
        $duration = "";
        $selectnewtimmer = "SELECT * FROM examinformation";
        $res = mysqli_query($conn, $selectnewtimmer);
        $row = mysqli_fetch_assoc($res);
        $duration = $row['minutesnew'];
        $hoursnew = $row['hoursnew'];
        
        $_SESSION['hoursnew'] = $hoursnew;
        $_SESSION['duration'] = $duration;
        $_SESSION['start_time'] = date("Y-m-d H:i:s");
        
        $endTime = date('Y-m-d H:i:s', strtotime('+'.$_SESSION['hoursnew']. 'hours' . $_SESSION['duration'] . 'minutes', strtotime($_SESSION['start_time'])));
            $_SESSION['endTime'] = $endTime;
        
        
        ?>
         <script type="text/javascript">
          
            window.

location="questions.php";
     </script>

This is my ‘response.php’ file

<?php 
session_start();

$form_time1 = date('Y-m-d H:i:s');
$to_time1 = $_SESSION['endTime'];

 $timefirst = strtotime($form_time1);
 
  $timeseconds = strtotime($to_time1);

    $diffecrceinseconds = $timeseconds - $timefirst;

  echo gmdate("H:i:s" , $diffecrceinseconds);



?>