这个Widgets API page on the Codex 有一个很好的扩展类的代码段（在本例中是WP\\u Widget类）。总的来说，这应该很清楚。确保您使用的是类的标准名称，并确保它是一个实际的类，而不仅仅是一个函数。

/**
 * Adds Foo_Widget widget.
 */
class Foo_Widget extends WP_Widget {

    /**
     * Register widget with WordPress.
     */
    public function __construct() {
        parent::__construct(
            \'foo_widget\', // Base ID
            \'Foo_Widget\', // Name
            array( \'description\' => __( \'A Foo Widget\', \'text_domain\' ), ) // Args
        );
    }

    /**
     * Front-end display of widget.
     *
     * @see WP_Widget::widget()
     *
     * @param array $args     Widget arguments.
     * @param array $instance Saved values from database.
     */
    public function widget( $args, $instance ) {
        extract( $args );
        $title = apply_filters( \'widget_title\', $instance[\'title\'] );

        echo $before_widget;
        if ( ! empty( $title ) )
            echo $before_title . $title . $after_title;
        ?>Hello, World!<?php
        echo $after_widget;
    }

    /**
     * Sanitize widget form values as they are saved.
     *
     * @see WP_Widget::update()
     *
     * @param array $new_instance Values just sent to be saved.
     * @param array $old_instance Previously saved values from database.
     *
     * @return array Updated safe values to be saved.
     */
    public function update( $new_instance, $old_instance ) {
        $instance = array();
        $instance[\'title\'] = strip_tags( $new_instance[\'title\'] );

        return $instance;
    }

    /**
     * Back-end widget form.
     *
     * @see WP_Widget::form()
     *
     * @param array $instance Previously saved values from database.
     */
    public function form( $instance ) {
        if ( isset( $instance[ \'title\' ] ) ) {
            $title = $instance[ \'title\' ];
        }
        else {
            $title = __( \'New title\', \'text_domain\' );
        }
        ?>
        <p>
        <label for="<?php echo $this->get_field_id( \'title\' ); ?>"><?php _e( \'Title:\' ); ?></label> 
        <input class="widefat" id="<?php echo $this->get_field_id( \'title\' ); ?>" name="<?php echo $this->get_field_name( \'title\' ); ?>" type="text" value="<?php echo esc_attr( $title ); ?>" />
        </p>
        <?php 
    }

} // class Foo_Widget

// register Foo_Widget widget
add_action( \'widgets_init\', create_function( \'\', \'register_widget( "foo_widget" );\' ) );