我编写了一段代码，用于使用ajax for WordPress从数据库中检索数据。以下代码是在functions.php. 代码用于获取类别id下的id和子类别名称，类别id定义为$cat . 结果，我需要显示为下拉列表

function fetchData(){
  global $wpdb;
    $catId = $_POST[\'key\']; // value from the ajax

        // Now we want to JSON encode these values to send them to $.ajax success.
      if($catId){
         $result_fromDB = $wpdb->get_results("SELECT * FROM  sub_category where categor_id = \'".$catId."\'");
          echo "<option value=\'".$catId."\' >".$catId."</option>"; //This line is returned in ajax. this is a test code
         foreach ($result_fromDB as $subcat) {
                           echo "<option value=\'".$subcat->id."\' >".$subcat->sub_category_name."</option>"; // This line doesnt returned, getting only **<option></option>** in console
          }
      }

        die();
    }

jQuery.ajax({
    type: \'POST\',
    url: "<?php echo admin_url( \'admin-ajax.php\' ); ?>",
    data: {
         \'key\' : catId,
        \'action\': "fetch_data" // very important
    },
    success : function (data) {
             console.log(data);
             jQuery(\'#sub_cat\').html(data);
            }
  });