I could have created the template error404.ctp under /views/errors/ and written any URL parsing functionality there but coding any PHP logic in a HTML template is not recommended in the MVC framework paradigm. It is worth noting that CakePHP allows you to create the following error templates under /views/errors/ that are great if you just want custom error pages without the need for any PHP code:
error404.ctp
missing_action.ctp
missing_component_class.ctp
missing_component_file.ctp
missing_connection.ctp
missing_controller.ctp
missing_helper_class.ctp
missing_helper_file.ctp
missing_layout.ctp
missing_model.ctp
missing_scaffolddb.ctp
missing_table.ctp
missing_view.ctp
private_action.ctp
scaffold_error.ctp


As I did have a requirement to write some PHP code I created the file app_error.php under the root directory:

class AppError extends ErrorHandler {
public function error404($arg) {
// If someone arrives here - let's keep the tracking codes
$url = $arg['url'];
$url_string = "";
// ...
$this->controller->redirect("/pages/mainpage/" . $url_string);
exit();
}
}

As can be seen, you just need to create a function named the same as the error. In this example I am not using the error404.ctp template after processing, instead redirecting to another static page with the tracking codes appended to the end of the URL.