Cada vez que envío el formulario de mi sitio web, solo crea una nueva ID. Pero todos los demás valores están vacíos, ¿qué debo hacer?

<?php
error_reporting(E_ALL);ini_set('display_errors',1); 

$servername = "localhost";
$username = "seamaszhou";
$password = "123456";
$dbname = "guest";




$conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, 
$password);
// set the PDO error mode to exception
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

// prepare sql and bind parameters
$stmt = $conn->prepare("INSERT INTO guest 
(guestName,guestEmail,guestContent) 
VALUES (:guestName, :guestEmail, :guestContent)");

$stmt->bindParam(':guestName', $guestName);
$stmt->bindParam(':guestEmail', $guestEmail);
$stmt->bindParam(':guestContent,', $guestContent);

// insert a row
$guestName = "$guestName";
$guestContent = "$guestContent";
$guestEmail = "$guestEmail";
$stmt->execute();



?>