I am typing a simple codes in PHP and HTML. How can I display the result in same page of browser not in another window tab or page?

Following is the simple HTML formula code:

To add two numbers and display on the same page

```
<html>
<head>
    <title> Sum of two numbers </title>
<body>

    <form action="sum.php" method="POST">
    First Number:  <input type="Text" Name="Num1"><br>
    Second Number: <input type="Text" Name="Num2"><p>
    <input type="Submit" value="Calculate">
    </form>

</body>
</html>

```

and `sum.php` file is as follows.

```
<?php
// add First and Second Numbers
$sum = $_POST["Num1"] + $_POST["Num2"];
// their sum is diplayed as
echo "The sum of  First Number (".$_POST["Num1"].") and
Second Number  (".$_POST["Num2"].") is $sum";
?>

```