How to compare post tags?

Working off the assumption that you’re already on Post One, and want to add Post Two below it, as a “Related Posts” type feature, you can use a standard WordPress function to get an array of One’s tags, then do a query to grab and display posts with tags that match.

//get just the IDs of Post One's tags
$tag_ids = wp_get_post_tags($post->ID, array('fields' => 'ids'));

//perform a query looking up any posts that have all the same tags.
//note: 'tag__and' will look for posts with ALL the tags, you can replace
//this with 'tag__in' if you want a less strict search to find ANY of the tags. 
$tag_query = new WP_Query( array( 'tag__and' => $tag_ids ) );

//standard loop fare to display posts that are found
if($tag_query->have_posts()) : while($tag_query->have_posts()) : $tag_query->the_post(); ?>
    <div class="related_post">
        <h3><?php echo get_the_title(); ?></h3>
        <?php the_content(); ?>
    </div>
<?php endwhile; endif;

//reinstate original WordPress query
wp_reset_query();