What is the quickest way to make a widget?

I have some PHP code that I would like to pull out of an existing page of a template, and put into a widget so I can move it around.

What is the quickest way to create a widget?

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The Widgets API is the quickest way to make a widget that’s reusable. Example use:

class My_Widget extends WP_Widget {
    /** 
      * PHP4 constructor
      */
    function My_Widget() {
        My_Widget::__construct();
    }

    /**
      * PHP5 constructor
      */
    function __construct() {
        // actual widget processes
    }

    /** 
      * Echo the settings update form
      *
      * @param array $instance Current settings
      */
    function form($instance) {
        // outputs the options form on admin
    }

    /** 
      * Update a particular instance
      *
      * @param array $new_instance New settings for this instance as input by the user via form()
      * @param array $old_instance Old settings for this instance
      * @return array Settings to save or bool (false) to cancel saving
      */
    function update($new_instance, $old_instance) {
        // processes widget options to be saved
    }

    /** 
      * Echo the widget content
      *
      * @param array $args Display arguments including before_title, after_title, before_widget, and after_widget
      * @param array $instance The settings for the particular instance of this widget
      */
    function widget($args, $instance) {
        // outputs the content of the widget
    }
}
register_widget('My_Widget');

If you don’t particularly care about having proper custom widget you can use a plugin like PHP Code Widget to run your code snippet