在$ .ajax帖子上没有设置$ _POST值

I'm new to PHP and javascript and I've run into a problem where I need to edit values in an xml document that is read in. I have an HTML Select that has been dynamically created by PHP code.

function outputTableRow() {
echo "<tr>";
    echo "<td>";
        echo "<select onchange=\"ValueChange(this);\">";
            echo "<option value=\"not value\" selected >No selection</option>";
            echo "<option value=\"A\" >A</option>";
            echo "<option value=\"B\" >B</option>";
            echo "<option value=\"C\" >C</option>";
            echo "<option value=\"D\" >D</option>";
        echo "</select>";
    echo "</td>";
echo "</tr>";
}

The onchange event for the select then calls a javascript function that (as far as my understanding goes) performs an ajax request to the _POST of that same php document.

function ValueChange(obj)
{
    var value = obj.value;

    $.ajax({
    type: "POST",
    url: "functions.php",
    data: "callFunction=UpdateValue"
    });
}

From there I try to check the value set by the ajax request in _POST to call the appropriate function.

if(isset($_POST['callFunction'])){
    if($_POST['callFunction'] == 'UpdateValue'){
       UpdateValue();
    }
}

And finally in the function I'm trying to get to, I'm trying to write a console log when that code is reached but it never get's there. In fact, the _POST is always empty.

function UpdateValue()
{
   echo '<script>console.log("VALUE CHANGE")</script>';
}

Any help is much appreciated.

First, a heredoc is how you should be outputting blocks of HTML:

<?php
function outputTableRow() {
    echo <<< HTML
<tr>
    <td>
        <select onchange="ValueChange(this);">
            <option value="not value" selected >No selection</option>
            <option value="A">A</option>
            <option value="B">B</option>
            <option value="C">C</option>
            <option value="D">D</option>
        </select>
    </td>
</tr>
HTML;
}

Then you'll want to add a success callback to your Ajax function:

function ValueChange(obj)
{
    var value = obj.value;

    $.ajax({
        type: "POST",
        url: "functions.php",
        data: "callFunction=UpdateValue",
        success: function(data) {$(body).append(data);}
    });
}

Really you should also be using jQuery to bind events instead of using onchange attribute as well. Assuming you gave the select id="changer" it would look something like this:

$("select#changer").on("change", function() {ValueChange(this);});

It's posible that the URL in Ajax call is bad, it must contain the server o something like that, you can try with ´URL: "http://localhost/functions.php"´