使用ajax调用，为什么我不能在php文档中使用这个$ var post数据？

Question

So...yeah, this is my function that calls ajax. I am trying to figure out why my $value isn't defined in my showuser.php??

function showUser2(value) {
                var xhr = new XMLHttpRequest();
                xhr.open("POST","showuser.php",true);
                xhr.onreadystatechange = function() {
                    if(xhr.readyState == 4 && xhr.status == 200) {
                        document.getElementById('container').innerHTML = xhr.responseText;
                    }
                }
                xhr.send("value="+value);
            }

showuser.php

<?php 
$value = $_POST['value'];
include_once("connect.php");
$obj = new User;
echo $value;
?>

This is how I call that javascript function ajax. I tried to put alert(value) in my function showUser2() and it gives exactly what I want...

<select id="selConc" onChange='showUser2(this.value);'>
                <option>Choisissez par nom:</option>
                <?php $obj->unConc(); ?>
            </select>

Answer 1

Try to add

xhr.setRequestHeader("Content-type","application/x-www-form-urlencoded");

before xhr.send(...).