如何在php中以表格的形式显示同一数据库中所选表中的数据？

<?php
// 连接数据库
$servername = "数据库服务器地址";
$username = "数据库用户名";
$password = "数据库密码";
$dbname = "数据库名称";

$conn = new mysqli($servername, $username, $password, $dbname);

// 检查连接是否成功
if ($conn->connect_error) {
    die("连接数据库失败: " . $conn->connect_error);
}

// 查询数据
$sql = "SELECT * FROM 表名";
$result = $conn->query($sql);

// 构建表格
echo "<table>";
echo "<thead><tr><th>列1</th><th>列2</th><th>列3</th></tr></thead>";
echo "<tbody>";

// 循环输出数据
if ($result->num_rows > 0) {
    while ($row = $result->fetch_assoc()) {
        echo "<tr>";
        echo "<td>" . $row["列1"] . "</td>";
        echo "<td>" . $row["列2"] . "</td>";
        echo "<td>" . $row["列3"] . "</td>";
        echo "</tr>";
    }
} else {
    echo "<tr><td colspan='3'>没有数据</td></tr>";
}

echo "</tbody>";
echo "</table>";

// 关闭数据库连接
$conn->close();
?>