如何在数据库文本框中显示基于php中combobox选定值的值？

<select id="comboBox" onchange="getData()">
  <option value="1">Option 1</option>
  <option value="2">Option 2</option>
  <option value="3">Option 3</option>
</select>
<input type="text" id="databaseText" readonly>

<script>
function getData() {
  var selectedValue = document.getElementById("comboBox").value;
  var xhr = new XMLHttpRequest();
  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
      document.getElementById("databaseText").value = xhr.responseText;
    }
  };
  xhr.open("GET", "getdata.php?selectedValue=" + selectedValue, true);
  xhr.send();
}
</script>

<?php
$selectedValue = $_GET["selectedValue"];

// 连接数据库，查询数据
$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";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
  die("连接失败: " . $conn->connect_error);
}

$sql = "SELECT value FROM your_table WHERE option = '$selectedValue'";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
  $row = $result->fetch_assoc();
  echo $row["value"];
} else {
  echo "无数据";
}

$conn->close();
?>