adapted modx foxycart script for processwire..

foxycart_xml_processor.php

<?

/*
	Title:
		FoxyCart_Inventory
	
	Description:
		Uses FoxyCart.com's XML Data Feeds to modify Processwire data.

	Version:
		1.0.0
		2013-11-22
*/


// ======================================================================================
// SET THE TEMPLATE VARIABLES and OTHERS
// *** MODIFY THIS SECTION ***
// ======================================================================================

// Finally, set your key to decrypt the XML that you receive from FoxyCart.
// This must match *exactly* what you entered at http://www.foxycart.com/admin
$key = 'api key';

// Optional: If you'd like to keep a log on your server of the results of this snippet, uncomment the
// following line, and put in an appropriate path.
// This is primarily intended for debugging when doing a custom implementation.
//$myFile = "/path/to/foxycart_datafeed.log";

// setup paths on server to the shared files 
// get these from http://modx.com/extras/package/foxycartinventory
$rc4cryptPath = "/path/to/class.rc4crypt.php";
$xmlParserPath = "/path/to/class.xmlparser_php5.php";

// Edit the section below for processwire specific stuff

// ===================================================================================================
// CHECK THE XML
// (do not modify past this point)
// ===================================================================================================

$output = '';

if (isset($_POST["FoxyData"])) {
	// ===================================================================================================
	// DECRYPT YOUR DATA
	// (do not modify)
	// ===================================================================================================
	// Decrypt the data using your $key
	
	include $rc4cryptPath;

	// Then decrypt the XML
	$FoxyData_encrypted = urldecode($_POST["FoxyData"]);
	$FoxyData_decrypted = rc4crypt::decrypt($key,$FoxyData_encrypted);

	// ===================================================================================================
	// PARSE YOUR XML
	// (do not modify)
	// ===================================================================================================
	// We now have a big fat XML that we can do whatever we want with.

		include $xmlParserPath;
		//Set up the parser object
		$data = new XMLParser($FoxyData_decrypted);
		//Work the magic...
		$data->Parse();

	// ===================================================================================================
	// MODIFY THE INVENTORY TV VALUE
	// ===================================================================================================
	//$output .= print_r($data, 1);
	// The XML is now a nice array called $data. Let's have fun.
	// First, loop through the transactions
	if (is_object($data)) {
		// First, loop through the transactions
		foreach ($data->document->transactions[0]->transaction as $transaction) {
			// Then through the products per transaction
			foreach ($transaction->transaction_details[0]->transaction_detail as $detail) {
				// Then get the codes
				if ($detail->product_code != '') {
					
					// Set the current code and quantity
					$code = $detail->product_code[0]->tagData;
					$output .= "code = $code \n";
					$quantity = $detail->product_quantity[0]->tagData;
					$output .= "quantity = $quantity \n";
				
					// First get the Product of the product code field
					$product = $pages->get("template=product-bstock, product_sku=$code");
					$output .= "Product = {$product->title} \n";			
					
					//Decrement the inventory by the quantity purchased
					
					if($product && ($product->stock_qty > 0)) {
						$curQty = $product->stock_qty;
						$newQty = $curQty - $quantity;
						$product->setOutputFormatting(false);
						$product->stock_qty = $newQty;
						$product->save();
					}

				}
			}
		}
		// That was fun.
		// Are we satisfied? If so, return "foxy" so these transactions can be marked as delivered.
		// Dont't forget, you can always export your data directly from the store admin page as well.
		
		//$output = "foxy";
		die("foxy");
	} else {
		// It's not an object, which means something went wrong (like an incorrect key).
		//$output = "error: not an object";
		die('error: not an object');
	}
	
} else {
	// If you're here, you didn't receive the FoxyData post, so error out.
	// This ensures that the transactions will still show up on the next datafeed.
	//$output = "error: no post data";
	die('error: no post data');
}


// ===================================================================================================
// WRITE THE RESULTS TO A FILE ON YOUR SERVER
// ===================================================================================================
if ($myFile) {
	$spacer = "===============\n";
	$date = date("Y-m-d H:i:s") . "\n";
	$fh = fopen($myFile, 'a') or die("can't open file");
	fwrite($fh, $spacer);
	fwrite($fh, $output);
	fwrite($fh, $date);
	fwrite($fh, $spacer);
	fwrite($fh, $FoxyData_decrypted);
	fclose($fh);
}


return $output;
echo $output;
?>