PHP HTML Code Question

I am doing some of the extra credit assignments in PHP "adding a contact form" Attached is the code plus my error message. know the error message is telling me that there null value represents a variable with no value. I just don't see where the error is. BTW it does work its just when I do $_POST my Payment Title shows up but the data field is empty.

<?php

$name = $_POST["name"];

$email = $_POST["email"];

$message = $_POST["message"];

$payment = $_POST["payment"];

$email_body = "";

$email_body = $email_body . "Name: " . $name . "\n";

$email_body = $email_body . "Email: " . $email . "\n";

$email_body = $email_body . "Message: " . $message . "\n";

$email_body = $email_body . "Payment: " . $payment;

echo $email_body

?>