I've read about how google is using XMLHTTPRequest and want to learn how I can implement XMLHTTPRequest using PHP. Any links or tutorials are much appreciated.
You really don't need to know PHP to do anything like Google Suggest. Basically you use XMLHttpRequest to get the file just like you would in a normal PHP application. Alternately, you can go echo '<script> parent.function(); </script>'; and use an iframe, which is the only alternative for RPC that supports more than just IE/Opera/Firefox.