ajax php file download and update mysql database field

I am starting to get to grips with ajax and need some assistance to code for form submission in which a mysql field value is updated and a file is downloaded. The principle of the routine is to checkout/download a file for revision and, having done so, ensure that nobody else can checkout the same file.

The first part of the problem I have done by simply using a standard php update function when the form is submitted. However, I am trying to use ajax to manage file download and I am having difficulty getting it configured.

I have attached a screenshot of the form page which is a list of radio buttons and a 'Checkout' (submit) button. When one of the radio buttons is selected and the button clicked, the database entry is updated to set the docCheckedOut field in the document table to true. At the same time a file download dialog should appear, and this isn't happening.

The pertinent parts of the code are shown below. All assistance gratefully received!

<!-- ajax functions all contained in lib.js-->
<!-- Process the download -->
function downloadFile(fileName)
{
	// Takes the file information and passes it to downloadFile.php
	xmlhttp = getxmlhttpObject();
	if (xmlhttp == null)
	{
		alert ("Browser does not support HTTP Request");
	 	return;
	}
	var serverPage = "../includes/downloadFile.php";
	obj = document.getElementById(fileName);
	processAjax(serverPage, obj, "post", "fileName");
}
 
<!-- process the ajax request -->
function processAjax(serverPage, obj, getOrPost, str)
{
	// Get an xmlhttpRequest object
	xmlhttp = getxmlhttpObject();
	if (getOrPost == "get")
	{
		xmlhttp.open("GET", serverPage);
		xmlhttp.onreadystatechange = function()
		{
			if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
				obj.innerHTML = xmlhttp.responseText;
		}
		xmlhttp.send(null);
	}
	else
	{
		xmlhttp.open("POST", serverPage, true)
		xmlhttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");
		xmlhttp.onreadystatechange = function()
		{
			if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
				obj.innerHTML = xmlhttp.responseText;
		}
		xmlhttp.send(str);
	}
}
 
<!-- get the xmlhttp object -->
function getxmlhttpObject()
{
	// Create a boolean to check for a valid MS IE browser instance
	var xmlhttp = false;
	
	// Check if IE is being used
	try
	{
		// If the js version is > 5
		xmlhttp=new ActiveXObject("Msxml2.XMLHTTP");
	}
	catch (e)
	{
		try
		{
			// If js version <= 5, then use the older activeX object
			xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
		}
		catch(E)
		{
			// Otherwise a non-IE browser is being used
			xmlhttp = false;
		}
	}
	
	// If not using IE, create a js instance of the object
	if (!xmlhttp && typeof XMLHttpRequest != 'undefined')
	{
		xmlhttp = new XMLHttpRequest();
	}
	
	return xmlhttp;
}
 
<!-- php file download routine (courtesy of Zubrag) -->
<?php
// downloadFile.php
// Download a file (with download resumption capability)
 
// Define the downloadable directory
define('BASE_DIR','http://MYDOMAIN/docsGeneral/');
 
/*
This function takes a path to a file to output ($file), 
the filename that the browser will see ($name) and 
the MIME type of the file ($mime_type, optional).
 
If you want to do something on download abort/finish,
register_shutdown_function('function_name');
*/
if(!is_readable($file)) die('File not found or inaccessible!');
	
$size = filesize($file);
$name = rawurldecode($name);
 
/* Figure out the MIME type (if not specified) */
$known_mime_types=array("pdf" => "application/pdf",
			"txt" => "text/plain",
			"html" => "text/html",
			"htm" => "text/html",
			"exe" => "application/octet-stream",
			"zip" => "application/zip",
			"doc" => "application/msword",
			"xls" => "application/vnd.ms-excel",
			"ppt" => "application/vnd.ms-powerpoint",
			"gif" => "image/gif",
			"png" => "image/png",
			"jpeg"=> "image/jpg",
			"jpg" =>  "image/jpg",
			"php" => "text/plain");
 
if ($mime_type=='')
{
$file_extension = strtolower(substr(strrchr($file,"."),1));
if(array_key_exists($file_extension, $known_mime_types)) $mime_type=$known_mime_types[$file_extension];
else $mime_type="application/force-download";
}
 
@ob_end_clean(); //turn off output buffering to decrease cpu usage
 
// required for IE, otherwise Content-Disposition may be ignored
if(ini_get('zlib.output_compression')) ini_set('zlib.output_compression', 'Off');
 
header('Content-Type: ' . $mime_type);
header('Content-Disposition: attachment; filename="'.$name.'"');
header("Content-Transfer-Encoding: binary");
header('Accept-Ranges: bytes');
 
/* The three lines below basically make the 
download non-cacheable */
header("Cache-control: private");
header('Pragma: private');
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
 
// multipart-download and download resuming support
if(isset($_SERVER['HTTP_RANGE']))
{
list($a, $range) = explode("=",$_SERVER['HTTP_RANGE'],2);
list($range) = explode(",",$range,2);
list($range, $range_end) = explode("-", $range);
$range=intval($range);
if(!$range_end) $range_end=$size-1;
else $range_end=intval($range_end);
$new_length = $range_end-$range+1;
header("HTTP/1.1 206 Partial Content");
header("Content-Length: $new_length");
header("Content-Range: bytes $range-$range_end/$size");
}
else 
{
$new_length=$size;
header("Content-Length: ".$size);
}
 
/* output the file itself */
$chunksize = 1*(1024*1024); //you may want to change this
$bytes_send = 0;
if ($file = fopen($file, 'r'))
{
if(isset($_SERVER['HTTP_RANGE'])) fseek($file, $range);
while(!feof($file) && (!connection_aborted()) && ($bytes_send<$new_length))
{
$buffer = fread($file, $chunksize);
print($buffer); //echo($buffer); // is also possible
flush();
$bytes_send += strlen($buffer);
}
fclose($file);
}
else die('Error - can not open file.');
 
die();

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form.jpg