非常感谢您在以下问题上的帮助。我有3个动态相关下拉列表。方案1在以下情况下工作：单击下拉列表（dd）1=>dd2中的值更改=>dd3中的值更改。但我需要dd3也依赖于dd1和dd2。我有ajax代码，假设它有错误：

$(document).ready(function(){
 $('#country', '#state').on('change',function(){
 var countryID = $('#country').val();
 var stateID = $('#state').val();
 if(countryID, stateID){
 $.ajax({
 type:'POST',
 url:'search_city.php',
 data: "country_id="+countryID+"&state_id="+stateID,
 success:function(html){
 $('#city').html(html);
 }
 });

 }else{

 $('#city').html('<option value="">Select state first</option>');

 }
 })
 });

添加php文件：

<?php 
  include('connect.php');  

  if (isset($_POST["country_id"] || isset($_POST["state_id"]))) {

  $query = $conn->query("SELECT * FROM city WHERE country_id = 
  ".$_POST['country_id']." AND state_id = ".$_POST['state_id']." ORDER BY 
  city_name");

  $rowCount = $query->num_rows;

  if($rowCount > 0){
    echo '<option value="">Select city</option>';
   while($row = $query->fetch_assoc()){
    echo '<option 
    value="'.$row['city_id'].'">'.$row['city_name'].'</option>';
    }
    }else{

   echo '<option value="">City not available</option>';
   }
   }

   ?>