PHP预处理语句执行过程中不知道什么问题，求大神解答，分数不多，下面是代码

$servername = 'servername'; //服务器名称$username = 'username'; //数据库用户名$password = 'password'; //登录密码$dbname = 'mydb';//创建连接$conn = new mysqli($servername, $username, $password, $dbname);//检测连接(PHP 5.2.9和5.3.0版本适用)/* if($conn->connect_error){	die('连接失败：' . $conn->connect_error);} *///检测连接(兼容更早版本)if(mysqli_connect_error()){	die('连接失败:' . mysqli_connect_error());}else{	$sql = "INSERT INTO myguests VALUES(?, ?, ?)";		//为mysqli_stmt_prepare()初始化statement对象	$stmt = mysqli_stmt_init($conn); 	echo $stmt; //不知道为什么$stmt是空的，没有数据		//预处理语句	if(mysqli_stmt_prepare($stmt, $sql)){		//绑定参数		mysqli_stmt_bind_param($stmt, 'sss', $firstname, $lastname, $email);				//设置参数并执行		$firstname = 'Sam';		$lastname = 'Poly';		$email = 'sam@gmail.com';		mysqli_stmt_execute($stmt);				$firstname = 'Mary';		$lastname = 'Moe';		$email = 'mary@gmail.com';		mysqli_stmt_execute($stmt);				$firstname = 'Julie';		$lastname = 'Dooley';		$email = 'julie@gmail.com';		mysqli_stmt_execute($stmt);				echo '插入成功';	}}