Untitled

<!DOCTYPE html>
    <html>
     <head>
  <title></title>
 </head>
  <body>

 <form method="POST">
  <button name="hey">submit</button>
  <p id="demo"></p>
  <?php

if (isset($_POST['button']))  {

  $severname = "localhost";
  $user = "";
  $password = "";
  $database = "";

   $con = mysqli_connect("$severname", "$user", "$password" , "$database");

   ?>


<script>

 var first = 20;
 var last = 90;
var sum  = first +last;

   document.getElementById("demo").innerHTML = sum;
   }
   </script>
   <?php  }?>
   </form>
    </body>
  </html>