When using a Javascript framework like MooTools or jQuery to execute an XMLHttpRequest, the XMLHttpRequest will usually be sent with a 'X-Requested-With' HTTP header. The Slim application will detect the HTTP request's ‘X-Requested-With’ header and flag the request as such. If for some reason an XMLHttpRequest cannot be sent with the 'X-Requested-With' HTTP header, you can force the Slim application to assume an HTTP request is an XMLHttpRequest by setting a GET, POST, or PUT parameter in the HTTP request named “isajax” with a truthy value.
Here's how you tell if the current request is an XHR/Ajax request:
$isXHR = $app->request()->isAjax(); //true or false
$isXHR = $app->request()->isXhr(); //true or false; alias of `isAjax()`