Register a Widget in Wordpress

custom_widget.php

<?php

class CustomWidget extends WP_Widget
{
    public function __construct()
    {
        $widget_options = array(
            'classname' => 'custom_widget',
            'description' => 'This is a custom widget',
        );
        parent::__construct('custom_widget', 'Halesi: Testimonial Widget', $widget_options);
    }

    public function widget($args, $instance)
    {
        if (empty( $instance )){
            return;
        };
        ?>
        <!--  Widget HTML  -->
        <?php echo $instance['title']; ?>    
        <?php
    }

    public function form($instance)
    {
        if (isset($instance['title'])) {
            $title = $instance['title'];
        } else {
            $title = __('New title', 'wpb_widget_domain');
        }
        ?>
        <p>
            <label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:'); ?></label>
            <input class="widefat" id="<?php echo $this->get_field_id('title'); ?>"
                   name="<?php echo $this->get_field_name('title'); ?>" type="text"
                   value="<?php echo esc_attr($title); ?>"/>
        </p>
        <?php
        return $instance;
    }

    public function update($new_instance, $old_instance)
    {
        $instance = array();
        $instance['title'] = (!empty($new_instance['title'])) ? strip_tags($new_instance['title']) : '';
        return $instance;
    }
}

function register_custom_widget()
{
    register_widget('CustomWidget');
}

add_action('widgets_init', 'register_custom_widget');