All helpers extend a special class, AppHelper (just like models extend AppModel and controllers extend AppController). To create functionality that would be available to all helpers, create /app/app_helper.php.

<?php
class AppHelper extends Helper {
    function customMethod () {
    }
}
?>

1. <?php
2. class AppHelper extends Helper {
3. function customMethod () {
4. }
5. }
6. ?>