wp-widget-example.php

<?php

class MyNewWidget extends WP_Widget {

 function __construct() {
  // Instantiate the parent object
  parent::__construct('mynewwidgetbaseid, 'My New Widget Title');
 }

 function widget($args, $instance) {
  // Widget output
 }

 function update($new_instance, $old_instance) {
  // Save widget options
 }

 function form($instance) {
  // Output admin widget options form
 }
}

function myplugin_register_widgets() {
 register_widget('MyNewWidget');
}

add_action('widgets_init', 'myplugin_register_widgets');