Undefined Index

<?php 
if(isset($_GET['r'])){
    $fullname= $_POST['fullname'];
    $username= $_POST['username'];
    $email= $_POST['email'];
    $message= $_POST['message'];
    $status= "pending";

    require ('inc/db_connect.php');
    $order ="INSERT INTO message (username, fullname, email, message, status)
             VALUES('$username','$fullname','$email','$message','$status')";
    $result=mysqli_query($con,$order);
    if($result){
        $feedback= "Your message had been sent, we will soon reply to your query.";
    } else {
        $feedback= "Something went wrong, please try again later.";
    }
    //mysqli_close($con);

}
    if(isset($_COOKIE['id'])){
        $id= $_COOKIE['id'];
        require ('inc/db_connect.php');

        $order= "SELECT name,username,email FROM persons WHERE id='$id'";
        $result= mysqli_query($con,$order);
        $row= mysqli_fetch_array($result);
        $full_name=$row['name'];
        $user_name=$row['username'];
        $email_=$row['email'];

        mysqli_close($con);
        if(isset($feedback)){
            echo $feedback;
        }
        ?>
        <form method="POST" action="support.php?r=send">
            <input type="text" name="fullname" id="fullname" value="<?php echo htmlspecialchars($full_name); ?>" >
            <input type="text" name="username" id="username" value="<?php echo htmlspecialchars($user_name); ?>" >
            <input type="email" name="email" id="email" value="<?php echo htmlspecialchars($email_); ?>" >
            <textarea name="message" id="message"></textarea>
            <input type="submit" value="Send">
        </form>

        <?php  }else { 
            if(isset($feedback)){
            echo $feedback;
        } ?>

        <form method="POST" action="support.php?r=send">
            <input type="text" name="fullname" id="fullname">
            <input type="text" name="username" id="username">
            <input type="email" name="email" id="email">
            <textarea name="message" id="message"></textarea>
            <input type="submit" value="Send">
        </form>
    <?php }
?>