从数据库中检索数据-未定义索引问题（ajax相关问题）

Question

我遵循了 w3schools 上的指南，但是当我从下拉菜单中选择一个用户时，我无法让脚本返回我想要的数据。我只是得到未定义的索引：userdrop 这是脚本（用 POST 更改了 GET，也许这就是我搞砸的）<script>function UserInfo(str) {    if (str == "") {        document.getElementById("txtHint").innerHTML = "";        return;    } else {        if (window.XMLHttpRequest) {            // code for IE7+, Firefox, Chrome, Opera, Safari            xmlhttp = new XMLHttpRequest();        } else {            // code for IE6, IE5            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");        }        xmlhttp.onreadystatechange = function() {            if (this.readyState == 4 && this.status == 200) {                document.getElementById("txtHint").innerHTML = this.responseText;            }        };        xmlhttp.open("POST","testing.php",true);        xmlhttp.send();    }}</script>下拉菜单：<form><select name="userdrop" onchange="UserInfo(this.value)">  <option value="">Select a person:</option>  <option value="1">Peter Griffin</option>  <option value="2">Lois Griffin</option>  <option value="3">Joseph Swanson</option>  <option value="4">Glenn Quagmire</option>  </select></form><br><div id="txtHint"><b>Person info will be listed here...</b></div>以及由于未定义索引而没有为我提供任何结果的 php 代码$row = $MMM->Users(intval($_POST['userdrop']));echo "<table><tr><th>ID</th><th>Name</th><th>City</th></tr><tr><td>".$row['user_id']."</td><td>".$row['user_name']."</td><td>".$row['user_city']."</td></tr></table>";

Answer 1

function UserInfo(str) {    //method starts with str as an argument

  var string = str;         //str is assigned to string.   

 var http = new XMLHttpRequest();  // create new HttpRequest instance 

 var url = "testing.php";          //the script to call to post data

 var params = 'id=string';          

 http.open("POST", url, true);

 //Send the proper header information along with the request

http.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');

 // Call a function when the state changes.

 http.onreadystatechange = function() {

    if(http.readyState == 4 && http.status == 200) {

        alert(http.responseText);

    }

 }

 http.send(params);

 }