我正在嘗試創(chuàng)建一個(gè)動(dòng)態(tài)下拉框，一旦做出選擇，該下拉框?qū)⒃L問MySQL。第二個(gè)框取決于第一個(gè)框的選擇。我對(duì)javascript不太熟悉，但是遇到了一些代碼，這些代碼似乎是我想要的，但是沒有執(zhí)行，我也不知道為什么。代碼的第一部分是javascript的選項(xiàng)字段，第二部分是dynamicdd.php。任何幫助將是巨大的。謝謝你。<tr>    <td>Country:  </td>    <td>        <select name="Countrybox" onchange="getlocation(this.value)">            <option value="none"> Please Select </option>            <?php              $qry2 = "Select Country from Locations";              $populate = mysqli_query($conn, $qry2);              while ($run = mysqli_fetch_assoc($populate)){                echo "<option value='".$run['Country']."'>".$run['Country']."</option>";              }            ?>        </select>    </td></tr><tr>    <td>Location:</td>    <td>        <select name="Locationbox" id="locationbycountry">            <option> Select Above First </option>        </select>    </td></tr><script type="text/javascript">function getlocation(locationarea) {    var xhttp = new XMLHttpRequest();    var url = "dynamicdd.php";    var data = new FormData();    data.append('SearchValue', locationarea);    xhttp.open('POST', url, true);    xhttp.send(data);    xhttp.onreadystatechange = function() {        if (xhttp.readyState == 4 && xhttp.status == 200) {            document.getElementById("locationbycountry").innerHTML = xhttp.responseText;        }    }}</script>dynamicdd.php<?phpif($_POST['SearchValue']){  $host = "localhost";  $username = "root";  $password = "";  $db = "Work";  $conn = mysqli_connect($host, $username ,$password, $db);  $choice = $_POST['SearchValue'];  $sql = "SELECT * FROM locations WHERE Country = '$choice'";  $result = mysqli_query($conn, $sql) or die(mysqli_error($conn));  while ($row = mysqli_fetch_assoc($result)){    echo "<option value='".$row['Location']."'>".$row['Location']."</option>";  }} ?>