wordpress: widget template from https://codex.wordpress.org/Widgets_API

wp_widget_template.php

// *** Extends the following class: src/wp-includes/class-wp-widget.php

class My_Widget extends WP_Widget {

	/**
	 * Sets up the widgets name etc
	 */
	public function __construct() {
		$widget_ops = array( 
			'classname' => 'my_widget',
			'description' => 'My Widget is awesome',
		);
		parent::__construct( 'my_widget', 'My Widget', $widget_ops );
	}

	/**
	 * Outputs the content of the widget
	 *
	 * @param array $args
	 * @param array $instance
	 */
	public function widget( $args, $instance ) {
		// outputs the content of the widget
	}

	/**
	 * Outputs the options form on admin
	 *
	 * @param array $instance The widget options
	 */
	public function form( $instance ) {
		// outputs the options form on admin
	}

	/**
	 * Processing widget options on save
	 *
	 * @param array $new_instance The new options
	 * @param array $old_instance The previous options
	 */
	public function update( $new_instance, $old_instance ) {
		// processes widget options to be saved
	}
}


// *** To register the widget:
add_action( 'widgets_init', function(){
	register_widget( 'My_Widget' );
});