Write a code for an automatic reply email. I can't get it to work, can someone tell me what's wrong with my commands?

Php code

// Define some constants

define( "RECIPIENT_NAME", "David Chandler" );

define( "RECIPIENT_EMAIL", "desiree@yahoo.com" );

define( "EMAIL_SUBJECT", "Message from Website" );

// Read the form values

$success = false;

$senderName = isset( $_POST['senderName'] ) ? preg_replace( "/[^\.\-\' a-zA-Z0-9]/", "", $_POST['senderName'] ) : "";

$senderEmail = isset( $_POST['senderEmail'] ) ? preg_replace( "/[^\.\-\_\@a-zA-Z0-9]/", "", $_POST['senderEmail'] ) : "";

$message = isset( $_POST['message'] ) ? preg_replace( "/(From:|To:|BCC:|CC:|Subject:|Content-Type:)/", "", $_POST['message'] ) : "";

// If all values exist, send the email

if ( $senderName && $senderEmail && $message ) {

$recipient = RECIPIENT_NAME . " <" . RECIPIENT_EMAIL . ">";

$headers = "From: " . $senderName . " <" . $senderEmail . ">";

$success = mail( $recipient, EMAIL_SUBJECT, $message, $headers );

}

// Return an appropriate response to the browser

if ( isset($_GET["ajax"]) ) {

echo $success ? "success" : "error";

} else {

?>

Thanks for sending your message! We'll get back to you shortly.

" ?>

There was a problem sending your message. Please try again.

" ?>

}

?>

and here is my html/css coding.

Name

Email

Message