如何在PHP，MYSQL中使用带有选择标签选项的搜索方法来搜索id，email，username？

<form method="GET" action="search.php">
  <label for="search">Search by:</label>
  <select name="search_by">
    <option value="id">ID</option>
    <option value="email">Email</option>
    <option value="username">Username</option>
  </select>
  <input type="text" name="keyword" placeholder="Keyword">
  <input type="submit" value="Search">
</form>

<?php
// 获取搜索条件和关键字
$searchBy = $_GET['search_by'];
$keyword = $_GET['keyword'];

// 连接到MYSQL数据库
$servername = "localhost";
$username = "your_username";
$password = "your_password";
$dbname = "your_database";

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

// 检查连接是否成功
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

// 构建SQL查询语句
$sql = "SELECT * FROM your_table WHERE $searchBy LIKE '%$keyword%'";

// 执行查询
$result = $conn->query($sql);

// 处理查询结果
if ($result->num_rows > 0) {
    while($row = $result->fetch_assoc()) {
        // 输出结果
        echo "ID: " . $row["id"]. " - Email: " . $row["email"]. " - Username: " . $row["username"]. "<br>";
    }
} else {
    echo "No results found.";
}

// 关闭数据库连接
$conn->close();
?>