Update- Blog moved to: http://saleemabdulhamid.com/blog/2009/12/server-communication-tutorial-for-280atlas-and-cappuccino
Link to original: http://www.youtube.com/watch?v=shP7GCfz6vY
The test.php server script just has the following code:
<?php$input=$_GET["input"];$output = "The number is: ".$input;echo $output;?>
Obviously, if you wanted, you could be accessing your database, etc. in the script and you can also echo JSON formatted data, and check the Cappuccino documentation for some methods that create CP arrays from JSON objects.