<?php
function modify_widget_categories($args){
  $include = "3,6,18"; // The IDs of the including categories
  $args["include"] = $include ;
 
  $args["hide_empty"] = 0; // If it is 0 it will show the empty categories as well
 
  return $args;
}
add_filter("widget_categories_args","modify_widget_categories");
?>

You can find the full list of parameters to modify the widget categories here: http://codex.wordpress.org/Function_Reference/get_categories#Default_Usage
.

So, say you’ve got some categories you use privately so you know which posts you used to talk about your IMAP email server, and which ones you used to talk about the times you play Foxy bingo (www.foxybingo.com). If you don’t want these categories to show up, but you do want “everyday”, “going out”, “thoughts on politics” and such to show without costing you the entire widget, there is a solution. Read on to learn how to sort this issue.

To exclude categories from WordPress default widget “Categories”, add this code in the file functions.php of the using theme:

<?php
function exclude_widget_categories($args){
  $exclude = "3,6,18"; // The IDs of the excluding categories
  $args["exclude"] = $exclude;
  return $args;
}
add_filter("widget_categories_args","exclude_widget_categories");
?>

The method to exclude the categories from the widget is better, as it is in the file of the theme and it will work after the updating WordPress versions.

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