widget_first_last_classes.php

<?php
/**
 * Add "first" and "last" CSS classes to dynamic sidebar widgets. Also adds numeric index class for each widget (widget-1, widget-2, etc.)
 */
function hip_widget_first_last_classes( $params ) {
  	
	global $my_widget_num; // Global a counter array
  
	$this_id = $params[0]['id']; // Get the id for the current sidebar we're processing
	$arr_registered_widgets = wp_get_sidebars_widgets(); // Get an array of ALL registered widgets	

	if( !$my_widget_num ) {
		// If the counter array doesn't exist, create it
		$my_widget_num = array();
	}

	if( !isset( $arr_registered_widgets[$this_id] ) || !is_array( $arr_registered_widgets[$this_id] ) ) {
		 // Check if the current sidebar has no widgets
		return $params; // No widgets in this sidebar... bail early.
	}

	if( isset( $my_widget_num[$this_id] ) ) {
		 // See if the counter array has an entry for this sidebar
		$my_widget_num[$this_id] ++;
	} else {
		 // If not, create it starting with 1
		$my_widget_num[$this_id] = 1;
	}
	
	// Add a widget number class for additional styling options
	$class = 'class="widget_' . $my_widget_num[$this_id] . ' ';

	if( $my_widget_num[$this_id] == 1 ) {
		// If this is the first widget
		$class .= 'widget_first ';
	} elseif( $my_widget_num[$this_id] == count( $arr_registered_widgets[$this_id] ) ) {
		// If this is the last widget
		$class .= 'widget_last ';
	}
	
	// Insert our new classes into "before widget"
	$params[0]['before_widget'] = str_replace( 'class="', $class, $params[0]['before_widget'] ); 

	return $params;

}
add_filter( 'dynamic_sidebar_params', 'hip_widget_first_last_classes' );