WP class to create custom widget for displaying similar posts

similar_posts_widget.php

<?php
class similar_posts_widget extends WP_Widget
{
    function __construct()
    {
        parent::__construct('similar_posts_widget', __('Similar Posts', 'custom-slug'), [
            'description' => __('Display similar post', 'custom-slug'),
        ]);
    }

    /**
     * @param $instance
     */
    public function form($instance)
    {
        /**
         * Set default widget title and number of posts to display.
         */
        $title           = __('Similar Posts', 'custom-slug');
        $number_of_posts = __('3', 'custom-slug');

        if (isset($instance['title']))
        {
            $title = $instance['title'];
        }

        if (isset($instance['number_of_posts']))
        {
            $number_of_posts = $instance['number_of_posts'];
        }

        /**
         * Output widget settings, allowing user to set custom Widget Title and Number of Posts.
         */
        ob_start();
        ?>
        <p>
          <label for="<?=$this->get_field_id('title');?>"><?php _e('Title:');?></label>
          <input id="<?=$this->get_field_id('title');?>" name="<?=$this->get_field_name('title');?>" type="text" value="<?=esc_attr($title);?>" />
          <label for="<?=$this->get_field_id('number_of_posts');?>"><?php _e('Number Of Posts:');?></label>
          <input id="<?=$this->get_field_id('number_of_posts');?>" name="<?=$this->get_field_name('number_of_posts');?>" type="number" value="<?=esc_attr($number_of_posts);?>" />
        </p>
        <?php
        echo preg_replace('/\s{2,}/im', '', ob_get_clean());  // Remove extra spaces from the final HTML output.
    }

    /**
     * @param  $new_instance
     * @param  $old_instance
     * @return mixed
     */
    public function update($new_instance, $old_instance)
    {
        /**
         * Remove any HTML tags from the user input and return the instance to be saved.
         */
        $instance                    = [];
        $instance['title']           = ( ! empty($new_instance['title'])) ? strip_tags($new_instance['title']) : '';
        $instance['number_of_posts'] = ( ! empty($new_instance['number_of_posts'])) ? strip_tags($new_instance['number_of_posts']) : '';
        return $instance;
    }

    /**
     * @param $args
     * @param $instance
     */
    public function widget($args, $instance)
    {
        global $post;
        $title = apply_filters('widget_title', $instance['title']);
        /**
         * Fetch the similar posts and output the results.
         * You can change div to ul, either way you have write your own CSS.
         */
        ob_start();
        echo $args['before_widget'];

        if ( ! empty($title))
        {
            echo $args['before_title'] . $title . $args['after_title'];
        }

        $tags = wp_get_post_tags($post->ID);

        if ($tags)
        {
            $tag_ids = [];

            foreach ($tags as $individual_tag)
            {
                $tag_ids[] = $individual_tag->term_id;
            }

            $wp_query = new WP_Query([
                'tag__in'             => $tag_ids,
                'post__not_in'        => [$post->ID],
                'posts_per_page'      => $instance['number_of_posts'],
                'ignore_sticky_posts' => 1,
            ]);
            ?>
            <div class="similar-posts">
              <?php

              while ($wp_query->have_posts())
              {
                $wp_query->the_post();
                ?>
                <div class="similar-post">
                  <a href="<?=the_permalink();?>" rel="external"><?=get_the_title();?></a>
                </div>
                <?
              }

              ?>
            </div>
            <?php

          }

          wp_reset_query();
        echo $args['after_widget'];
        echo preg_replace('/\s{2,}/im', '', ob_get_clean()); // Remove extra spaces from the final HTML output.
    }

}

function wpb_load_widget()
{
    register_widget('similar_posts_widget');
}

add_action('widgets_init', 'wpb_load_widget');