Widget

<?php
// Register and load the widget
add_action( 'widgets_init', 'wpb_load_widget' );
function wpb_load_widget() {
    register_widget( 'featured_product_widget' );
}

// Creating the widget
class featured_product_widget extends WP_Widget {
    function __construct() {
        parent::__construct(
            // Base ID of your widget
            'featured_product_widget',
            // Widget name will appear in UI
            __('Uitgelichte product', 'mediasoep'),
            // Widget description
            array( 'description' => __( 'Kies hier een product die je wilt tonen.', 'mediasoep' ), )
        );
    }

    /**
    * Outputs the content of the widget
    *
    * @param array $args
    * @param array $instance
    */
    public function widget( $args, $instance ) {
        // Outputs the content of the widget
        if ( ! isset( $args['widget_id'] ) ) {
            $args['widget_id'] = $this->id;
        }

        // Widget ID with prefix for use in ACF API functions
        $widget_id = 'widget_' . $args['widget_id'];

        $product = get_field( 'product', $widget_id );

        echo $args['before_widget'];
        echo '<pre>';
        echo var_dump( $product );
        echo '</pre>';
        echo $args['after_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
     *
     * @return array
     */
    public function update( $new_instance, $old_instance ) {
        // Processes widget options to be saved
    }
} // Class featured_product_widget ends here