How to include in a php widget

Hi,

i try to include the subtitles in a “see other posts” widget. This is the code of the widget:

<?php
if (is_single()) {
$post_ID = $wp_query->posts[0]->ID;
$all_cats_of_post = get_the_category($post_ID);
$first_cat_ID = $all_cats_of_post[0]->cat_ID;
$first_cat_name = $all_cats_of_post[0]->cat_name;
?>
<div id="category-posts-<?php echo $first_cat_ID; ?>" class="widget widget_recent_entries">
  <h2>Weitere Beitr?ge aus <?php echo $first_cat_name; ?></h2>
    <div>
        <ul>
        <?php global $post; $cat_posts = get_posts('numberposts=20&category='.$first_cat_ID);
        foreach($cat_posts as $post) : ?>
            <li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
        <?php endforeach; ?>
        </ul>
    </div>
</div>
<?php } ?>

Could you, however, help me, what i have to write in this code to make the subtitels work?

thanks a lot in advanced!

https://www.ads-software.com/plugins/subtitles/