php 输出mysql错误

<?php
$servername = "localhost";
$username = "your_username";
$password = "your_password";
$dbname = "your_database";

// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);

// 检查连接
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 执行查询
$sql = "SELECT id, firstname, lastname FROM MyGuests";
$result = $conn->query($sql);

if (!$result) {
    echo "查询错误: " . $conn->error;
} else {
    // 输出结果
    if ($result->num_rows > 0) {
        while($row = $result->fetch_assoc()) {
            echo "id: " . $row["id"]. " - Name: " . $row["firstname"]. " " . $row["lastname"]. "<br>";
        }
    } else {
        echo "0 结果";
    }
}

// 关闭连接
$conn->close();
?>