Why id is Undefined here?

I have a delete.php too delete row data which is working fine. But in my error log I ge this error.

A session had already been started - ignoring session_start() in /home/delete.php on line 2
PHP Notice:  Constant DB_SERVER already defined in /home/db.php on line 4
PHP Notice:  Constant DB_USERNAME already defined in /home/db.php on line 5
PHP Notice:  Constant DB_PASSWORD already defined in /home/db.php on line 6
PHP Notice:  Constant DB_DATABASE already defined in /home/db.php on line 7
Undefined index: id in /home/delete.php on line 7

My delete.php is

<?php
session_start();
include("db.php"); //Establishing connection with our database

$dltsuccess="";

$DltQuery="DELETE from properties where id=".$_REQUEST['id'];
$result=mysqli_query($db, $DltQuery);

if($result)
    {

     echo "Record Deleted Successfully";
     echo "<script>setTimeout(\"location.href = 'admin.php';\",300);</script>";
     $dltsuccess= "Record Deleted Successfully";
    }
?>