app.js

var proxy = 'http://localhost/foobar/proxy.php';

$('form#new_customer').submit(function(){
  data =  '<customer>';
  data += '<name>' + $('#customer_name').val() +'</name>';
  data += '</customer>';
  $.ajax({
    url: proxy + '?api_path=customers.xml',
    contentType: 'application/xml',
    type: 'POST',
    data: data,
    dataType:'xml',
    beforeSend: function(){
      if ($.trim($('#customer_name').val()) == '')
        return false
    },
    success: function(data){
      alert('Created customer with customer number: ' + $(data).find('customer-number').text());
    }
  })
  return false;
})

proxy.php

<?php

  // Loosely based on the Jason Levitt PHP Proxy example for Yahoo! Web services.
  
  define ('HOSTNAME', 'http://foobar.localhost.com:3000/');

  $path = ($_POST['api_path']) ? $_POST['api_path'] : $_GET['api_path'];

  // Build url and add any parameters
  $url = HOSTNAME.$path.'?';
  while ($element = current($_GET)) {
		$url .= key($_GET).'='.$element.'&';
		next($_GET);
	}

  $ch = curl_init($url);
  curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_BASIC); 
  curl_setopt($ch, CURLOPT_USERPWD, 'abc123:X');
  curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/xml'));
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
  if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $HTTP_RAW_POST_DATA);
  }

  $xml = @curl_exec($ch);
  curl_close($ch);

  header("Content-Type: text/xml");
  echo $xml;
  
?>