Detect an AJAX Request in PHP

I like using the same PHP script for both AJAX and non-AJAX content requests. Using one script just makes everything easier because it's only one file to update/edit and it's one more cache-able request. One way to try detect an AJAX request (as opposed to a regular page load) is by using the following PHP code:

/* 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.... */

$_SERVER['HTTP_X_REQUESTED_WITH'] is the golden ticket but not all servers provide this variable so having other checks in place will be important.