I am reading the Drupal api docs. I'm looking under functions. At the end there are a lot of functions that start with an underscore. Some of them (all?) seem to be something called "helper functions." I've looked in my php books, searched drupal.org and on Google for "helper functions" mixed with "underscore" and "php." Came up with no answers. What's special about the underscore and functions that start with it? What does the underscore mean?