Email form using PHP doesn't work!

ok, so I followed the tutorial by senocular here on kirupa, but it doesn’t send the email right…
This is my PHP-script:

$sendTo = "[email protected]";
$subject = "My Flash site reply";
$headers = "From: " . $_POST["firstName"] ." ". $_POST["lastname"] . "<" . $_POST["email"] .">
$headers .= "Reply-To: " . $_POST["email"] . "
$headers .= "Return-path: " . $_POST["email"];
$message = $_POST["message"];
mail($sendTo, $subject, $message, $headers);

and the swf, php and fla file is here:

I don’t know the first thing about PHP, and I dont’ really get why there’s firstname and lastname, but I hope that you can help me sort that out aswell! I don’t think that those are needed, but I don’t know how much to delete from it…

EDIT: the .fla can’t be downloaded from there, so it’s here: