PHP4 compatibility patch for notifications_ui

I'm running Drupal 5.7 with PHP 4.3.10. And get the following error message when subscribing to a content type using the Notifications UI.
Fatal error: Call to undefined function: array_diff_key() in /sites/all/modules/notifications/notifications_ui/notifications_ui.module on line 283
This patch provides a function with backwards compatibility for array_diff_key