Need help with a contact form

Hi, I have a problem with my .php Contact Form. When it reaches my email, the name, last name, email is empty.

This is the code:


<?php
$first_name = $_POST['first_name'];
$last_name = $_POST['last_name'];
$visitor_email = $_POST['email'];
$message = $_POST['message'];


$email_from = 'myemail@hotmail.com';

$email_subject = "S&P Website";

$email_body = "First Name: $first_name.\n".
"Last Name: $last_name.\n".
"User Email: $visitor_email.\n".
"User Message: $message.\n";


$to = "mysecondemail@hotmail.com";

$headers = "From $email_from \r\n";

$headers .= "Reply-To: $visitor_email \r\n";

mail($to,$email_subject,$email_body,$headers);

header("Location: index.html");


?>