/* decide what the content should be up here .... */
$content = get_content(); //generic function;

/* AJAX check  */
if(!empty($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest') {
  /* special ajax here */
  die($content);
}

/* not ajax, do more.... */

http://davidwalsh.name/detect-ajax

Test for AJAX Header in PHP

ajax, php, header, jquery

PHP