如何将xmlhttp.responseText插入已存在的下拉html元素中？

Question

目标：我想从Connection.php片段中的getFaculties（）中检索值并将其插入到表单上的server { listen 80; server_name *.example.com; .... }

我的表格代码

<select>

这是执行请求的功能......

<form id="myform" method="GET">
  <label>Varsity Name</label><br>
  <select name="selVarsityName" id="selVarsityName" onchange="dropUpdate();">
  <?php
    require 'Connection.php';
    $conn->getVarsities();
  ?>                
  </select><br>

  <label>Faculty Name</label>
  <select name="selFaculty" id="selFaculty">
  </select><br>

这是Middle.php页面上的代码。

function dropUpdate() {
  if (window.XMLHttpRequest) {
    xmlhttp = new XMLHttpRequest();
  } else {
    xmlhttp = new ActiveXObject('Microsoft.XMLHTTP');
  }
  xmlhttp.onreadystatechange = function() {
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
      document.getElementById('selFaculty').innerHTML = xmlhttp.responseText;
    }
  };
  var ghgh = document.getElementById('selVarsityName').value;
  xmlhttp.open('GET', 'Middle.php?selVarsityName=' + ghgh, true);
  xmlhttp.send();

}

我的Connection.php页面上的代码

require './Connection.php';

if (isset($_GET['selVarsityName'])) {
  $selVarsityName = $_GET['selVarsityName'];
  $conn->getFaculties($selVarsityName);
}