同源策略 - JavaScript调用PHP

I know that this is a popular topic, but I've yet to find an answer that's completely comprehensive.

I'm trying to create a simple way for our 'customers' to place a Google Map on their website, which plots the position of our customers (or a subset thereof) on the map. The customers are in a MySQL database which is turned into XML on-the-fly by a PHP script (as per Google's example). This works fine on my website, but when I try it on another website the xmlHTTPRequest is not allowed to look at the PHP as it's on another domain.

I can circumvent this by writing another PHP file on the other domain which simply reads the PHP file on the original domain. But not all our customers will have PHP running on their servers. Is there any way that I can return the XML results from our database using JavaScript?

A couple of points:

1. The JavaScript that makes the xmlHTTPRequest still sits on our server -- our clients link to it from a script tag. I thought that might be enough, but the 'origin' (according to Chrome, anyway) is still seen as domain#2

2. This is great: if I use an absolute reference in the xmlHTTPRequest (e.g. request.open('GET', 'http://mydomain.com/api/foo.php', true)) then it will fail in IE, but if I use a relative reference ('/api/foo.php') it will work.

3. I don't know enough about it, but could I use JSON? I've seen: 'script src="http://..../someData.js?callback=some_func"' but don't know how, I would make 'someData.js' look like JSON? (I'm thinking very much in terms of functions, which probably is incorrect?).

4. I've tried adding: header("Access-Control-Allow-Origin: *"); to the top of the PHP that outputs the XML, but it's not really doing much that I can tell!

5. If I do use a PHP wrapper on the client's server, what's the advantage of using a cURL request, rather that simple file_get_contents or fopen?

Sorry, lots of questions, but any guidance would be greatly appreciated.

Massive thanks,

Mat