Allows the ability to add custom classes to WordPress widgets (just through filter, does not add form.)

wp-widget-custom-classes.php

<?php
//Hook into dynamic_sidebar_params filter
//found in dynamic_siderbar ( wp-include/widgets.php ) line 886
add_filter( 'dynamic_sidebar_params', 'extend_widgets_sidebar_params', 10);

//I have my classes stored within an array in an option called widget_classes
if((!$widget_classes = get_option('widget_classes')) || !is_array($widget_classes) ) $widget_classes = array();


/* Params callback on setting widget params */
function extend_widgets_sidebar_params($params) {
  //Call our globals
  global $widget_classes, $wp_registered_widgets;

	$widget_id = $params[0]['widget_id'];

	if(!isset($widget_classes[$widget_id]) || $widget_classes[$widget_id] == '')
		return $params;
	
  //The filter is called *after* WordPress sets the before_widget value, so easiest solution to str_replace to add in our classes
	$before_widget_string = $params[0]['before_widget'];
	$before_widget_replace = 'class="' . $widget_classes[$widget_id] .' ';
	$params[0]['before_widget'] = str_replace('class="', $before_widget_replace, $before_widget_string);

	return $params;
}
?>