此外，如果您希望对第一个和最后一个小部件进行操作，可以使用下面的代码。我把nautilus7的代码和MathSmath+durin的代码结合在一起http://wordpress.org/support/topic/how-to-first-and-last-css-classes-for-sidebar-widgets 要针对特定的侧栏，请根据侧栏中的小部件数量添加跨类，然后向组的第一个和最后一个小部件添加自定义类。

function cur_target_sidebar_add_classes_to_params($params) {

global $my_widget_num; // Global a counter array
$sidebar_id = $params[0][\'id\'];

if ( $sidebar_id == \'sidebar\' ) {

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

    $number_of_widgets = count($registered_widgets[$sidebar_id]);
    $rounded_number_of_widgets = floor(12 / $number_of_widgets); //Rounds number of widgets down to a whole number

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

    if(isset($my_widget_num[$sidebar_id])) { // See if the counter array has an entry for this sidebar
        $my_widget_num[$sidebar_id] ++;
    } else { // If not, create it starting with 1
        $my_widget_num[$sidebar_id] = 1;
    }

    $classes = \'span\' . $rounded_number_of_widgets;

    if($my_widget_num[$sidebar_id] == 1) { // If this is the first widget
        $classes .= \' first-widget \';
    } elseif($my_widget_num[$sidebar_id] == count($registered_widgets[$sidebar_id])) { // If this is the last widget
        $classes .= \' last-widget \';
    }

    $params[0][\'before_widget\'] = preg_replace(\'/class=\\"/\', \'class="\' . $classes . \' \', $params[0][\'before_widget\'], 1);
    }

    return $params;
}
add_filter(\'dynamic_sidebar_params\',\'cur_target_sidebar_add_classes_to_params\');

function wpse_54162_get_widgets_count( $sidebar_index ) {
    global $wp_registered_sidebars;

    $index = "sidebar-{$sidebar_index}";

    $sidebars_widgets = wp_get_sidebars_widgets();
    if ( empty($wp_registered_sidebars[$index]) || !array_key_exists($index, $sidebars_widgets) || !is_array($sidebars_widgets[$index]) || empty($sidebars_widgets[$index]) )
        return 0;

    return count( (array) $sidebars_widgets[$index] );
}

让我们尝试以下代码

function sidebar_widget_init()
    {
        global $_wp_sidebars_widgets;
        if ( empty( $_wp_sidebars_widgets ) ) :
            $_wp_sidebars_widgets = get_option( \'sidebars_widgets\', array() );
        endif;

        $sidebars_widgets_count = $_wp_sidebars_widgets;
        $sidebar_count = count( $sidebars_widgets_count[ \'sidebar3\' ] );

        if($sidebar_count == 2){
            $sidebar_class = \'class_name1\';
        } elseif($sidebar_count == 3){
            $sidebar_class = \'class_name2\';
        } elseif($sidebar_count == 4){
            $sidebar_class = \'class_name3\';
        }
        register_sidebar(array( \'name\'          => __( \'Sidebar Footer\', \'templatemesh\' ),
                                \'id\'            => \'sidebar3\',
                                \'description\'   => __( \'Widgets in this area will be shown on footer Sidebar.\', \'templatemesh\' ),
                                    \'before_title\'  => \'<h3  class="widget_title">\',
                                    \'after_title\'   => \'</h3>\',
                                    \'before_widget\' => \'<div class="\'.$sidebar_class.\'"><div id="%1$s" class="widget %2$s" >\',
                                    \'after_widget\'  => \'</div></div>\'
                    )
        );
    }
    add_action(\'widgets_init\',\'sidebar_widget_init\');