AJAX without jQuery is not sending POST data to PHP file

I've been trying to get an ajax alert layer to work with a POST method for several days and I can't come up with a reason for it not working. I use the same basic code to send form data through ajax with POST on other admin pages without trouble but when I try to send data that does not come from a form nothing gets to the server in $_POST.

Here's the flow of the code...

I use variables on a page like these:

$alertLayer = 1;
$autoCloseAlertLayer = 1;
$addAlertLayerCloseButton = 1;
$alertLayerMessage = $alertLayerMessage . '<h1>Test</h1><p>3rd test of the alert layer module.</p>';
$redirect = 0;
$redirectTo = 0;

and I include a script that calls a function at the bottom of the page like this:

if ($alertLayer == true)
{   
    echo "Lib.ajaxAlertFunction('/Modules/AlertLayer', $autoCloseAlertLayer, $addAlertLayerCloseButton, '$alertLayerMessage', $redirect, '$redirectTo');";
}

Here's the script that gets called:

Lib.ajaxAlertFunction = function (senturl, autoClose, closeButton, message, redirect, redirectTo)
{   
var ajaxRequest;

try
{
    ajaxRequest = new XMLHttpRequest();
}
catch (e)
{
    try
    {
        ajaxRequest = new ActiveXObjext("Msxml2.XMLHTTP");
    }
    catch (e)
    {
        try
        {
            ajaxRequest = new ActiveXObjext("Microsoft.XMLHTTP");
        }
        catch (e)
        {
            alert ("Your browser can't handle the truth!");
            return false;
        }
    }
}

if (!senturl)
{
    return false;
}
else
{
    // var data = encodeURIComponent("autoClose=" + autoClose + "&closeButton=" + closeButton + "&message=" + message + "&redirect=" + redirect + "&redirectTo=" + redirectTo);
    var data = "autoClose=" + autoClose + "&closeButton=" + closeButton + "&message=" + message + "&redirect=" + redirect + "&redirectTo=" + redirectTo;
}

ajaxRequest.onreadystatechange = function()
{
    if (ajaxRequest.readyState == 4 && ajaxRequest.status == 200)
    {
        document.getElementById('outerFrame').innerHTML += ajaxRequest.responseText;

        newAlertLayer = document.getElementById('alertLayer');
        var arr = newAlertLayer.getElementsByTagName('script')
        for (var n = 0; n < arr.length; n++)
        {
            eval(arr[n].innerHTML)
        }
    }
}
ajaxRequest.open('POST', senturl, true);
ajaxRequest.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
ajaxRequest.send(data);
}

NOTE: I have no problem sending this data with a 'GET' method but then a long message gets cut off. I have also tried to set up the 'data' variable in several different methods that I've searched over the past 3 days with no success.

The code that expects $_POST data goes as follows:

<?php
$ROOT = $_SERVER['DOCUMENT_ROOT'];
?>

/Modules/AlertLayer/alertLayer.css">
/Modules/AlertLayer/alertLayer.js

 $value)
{
    echo "
" . $key . " = " . $value . "
";
}
foreach ($_GET as $key => $value)
{
    echo "
" . $key . " = " . $value . "
";
}
?>

</div>

What am I missing that is different from sending form data and the GET method that is working with this code?
Again, GET is working but not sufficient, POST = no data at all.