Determine if WordPress AJAX request is a backend of frontend request

AJAX endpoints being natively supported in WordPress is awesome. Not being able to separate a frontend request (coming from a theme/frontend plugin) from a backend request (coming from the admin panel) is not as awesome.

Here’s a function to determine if a request is coming from the frontend or backend. It uses the HTTP referer to determine which URL the incoming call to admin-ajax.php originated from. If it originated from a /wp-admin/ url, we know for sure that it was a backend request.

function request_is_frontend_ajax()
{
  $script_filename = isset($_SERVER['SCRIPT_FILENAME']) ? $_SERVER['SCRIPT_FILENAME'] : '';

  //Try to figure out if frontend AJAX request... If we are DOING_AJAX; let's look closer
  if((defined('DOING_AJAX') && DOING_AJAX))
  {
          //From wp-includes/functions.php, wp_get_referer() function.
          //Required to fix: https://core.trac.wordpress.org/ticket/25294
          $ref = '';
          if ( ! empty( $_REQUEST['_wp_http_referer'] ) )
              $ref = wp_unslash( $_REQUEST['_wp_http_referer'] );
          elseif ( ! empty( $_SERVER['HTTP_REFERER'] ) )
              $ref = wp_unslash( $_SERVER['HTTP_REFERER'] );

    //If referer does not contain admin URL and we are using the admin-ajax.php endpoint, this is likely a frontend AJAX request
    if(((strpos($ref, admin_url()) === false) && (basename($script_filename) === 'admin-ajax.php')))
      return true;
  }

      //If no checks triggered, we end up here - not an AJAX request.
      return false;
}

Usage

if(request_is_frontend_ajax()) {
    //Do something only on frontend ajax
}