Update post meta (php basic question)

Hello.

Thank you, it works.

But how would I save as a value result of this whole function?

  function show_me_synonyms_and_similar_words()
    {

            //query
            global $post;
            $postid = get_the_ID();
            $posttags = get_the_tags();
            //get synonyms
            if ($posttags)
            {
                foreach ($posttags as $tag)
                {
                    $slug = $tag->slug . " ";
                }
                $output .= $slug;
            }

            $args = ["post__not_in" => [$postid], "post_status" => "publish", "orderby" => "title", "order" => "ASC", "tag_slug__and" => $output, ];

            $myposts = get_posts($args);
            if ($posttags)
            {
                echo "<h2 class='sym cust_text'>Synonyma:</h2>";
                echo '<ul class="syn tab">';

                foreach ($myposts as $post): ?>
		<li>
            <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
        </li> 
	
	<?php
                endforeach;
                wp_reset_postdata();
                echo "</ul>";
            }
        

        //search similar words by keyword    
        $field_c = str_replace(' ', '', strtolower(get_field("search_core")));
        $field_exclude = str_replace(' ', '', strtolower(get_field("search_core2")));
        if ($field_c)
        {
            $actual_title = str_replace(' ', '', strtolower(get_the_title()));
            $s = get_search_query();
            $args = ["s" => $field_c, ];
            // The Query          
            $the_query = new WP_Query($args);
            if ($the_query->have_posts())
            {
                $post_count = $the_query->found_posts;
                if (str_contains(str_replace(' ', '', strtolower($actual_title)) , $field_c) && $post_count == 1)
                {
                }
                else
                {
                    _e("<h2 class='sim'>Podobná slova:</h2>");
                    echo "<ul>";
                    while ($the_query->have_posts())
                    {
                        $the_query->the_post();
                        if (str_replace(' ', '', strtolower(get_the_title())) !== $actual_title)
                        {

                            if (strpos(str_replace(' ', '', strtolower(get_the_title())) , $field_exclude) == false)
                            {
?>      
                 
                    <li>
                        <a class="sim_terms" href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
                    </li>
						<?php
                            }
                        }
                    }
                    echo "</ul>";
                }
            }
        }
    }