I have an idea here that may seem a bit strange but it would work.
If you have ever worked with RSS feeds or AJAX what I am about to suggest is based on how they work.
When you display an RSS feed on your website technically what you are doing is displaying data from another servers database.
When you use AJAX you are also displaying data from an external source.
In both cases an undisclosed URL is being requested and the returned data is processed and displayed.
The way I suggest you handle this is have the php on the one server send data to the other sever using either cURL or GET values in a file request (fopen). Obviously if security is a concern apply a custom hash to all data values. The script, you are sending the data to, can then return any data via XML that the first php script can then easily process.
Its not one script getting the data directly from the other, but it is the same results you just have to use an intermediary.