<?php
class My_Widget extends WP_Widget {

    public function __construct() {
        $widget_ops = array(
            'classname' => 'my_widget',
            'description' => __( 'Description.' ),
            'customize_selective_refresh' => true,
        );
        $control_ops = array(
            'width' => 400,
            'height' => 350,
        );
        parent::__construct( 'my_widget', __( 'My Widget' ), $widget_ops, $control_ops );
    }

    public function widget( $args, $instance ) {
      extract( $args );
      $checkbox1 = ! empty( $instance['checkbox1'] ) ? $instance['checkbox1'] : false;
      $checkbox2 = ! empty( $instance['checkbox2'] ) ? $instance['checkbox2'] : false;
      $checkbox3 = ! empty( $instance['checkbox3'] ) ? $instance['checkbox3'] : false;

      echo $args['before_widget'];
      echo "Hello world, just testing this out.";
      echo $args['after_widget'];
    }

    public function form( $instance ) {

      $defaults = array(
        'checkbox1' => false, // not checked by default
        'checkbox2' => true,  // checked by default
        'checkbox3' => true,  // checked by default
      );

      $instance = wp_parse_args( (array) $instance, $defaults );
      ?>

    <p>
      <input
        class="checkbox"
        id="<?php echo esc_attr( $this->get_field_id( 'checkbox1' ) ); ?>"
        name="<?php echo esc_attr( $this->get_field_name( 'checkbox1' ) ); ?>"
        type="checkbox"
        <?php checked( '1', $instance['checkbox1'] ); ?>
        value="1">

      <label for="<?php echo esc_attr( $this->get_field_id( 'checkbox1' ) ); ?>">
        <?php _e( 'Checkbox #1', 'test' ); ?>
      </label>
    </p>

    <p>
      <input
        class="checkbox"
        id="<?php echo esc_attr( $this->get_field_id( 'checkbox2' ) ); ?>"
        name="<?php echo esc_attr( $this->get_field_name( 'checkbox2' ) ); ?>"
        type="checkbox"
        <?php checked( '1', $instance['checkbox2'] ); ?>
        value="1">

      <label for="<?php echo esc_attr( $this->get_field_id( 'checkbox2' ) ); ?>">
        <?php _e( 'Checkbox #2', 'test' ); ?>
      </label>
    </p>

    <p>
      <input
        class="checkbox"
        id="<?php echo esc_attr( $this->get_field_id( 'checkbox3' ) ); ?>"
        name="<?php echo esc_attr( $this->get_field_name( 'checkbox3' ) ); ?>"
        type="checkbox"
        <?php checked( '1', $instance['checkbox3'] ); ?>
        value="1">

      <label for="<?php echo esc_attr( $this->get_field_id( 'checkbox3' ) ); ?>">
        <?php _e( 'Checkbox #2', 'test' ); ?>
      </label>
    </p>

      <?php
    }

    public function update( $new_instance, $old_instance ) {

      $instance = $old_instance;

      $instance['checkbox1'] = isset( $new_instance['checkbox1'] ) ? 1 : false;
      $instance['checkbox2'] = isset( $new_instance['checkbox2'] ) ? 1 : false;
      $instance['checkbox3'] = isset( $new_instance['checkbox3'] ) ? 1 : false;

      return $instance;
    }
}