Related posts without plugin;What I’m doing wrong ?

I’m trying to retrieve the tags from the main loop, and, instead of print them, get the titles of all other posts matching those tags.

My code:

<?php
$posttags = get_the_tags();
?>

<?php
 $posts = get_posts('tag=$posttags');
 foreach($posts as $post):
    setup_postdata($post);
?>
<a href="<?php the_permalink(); ?>"><h4><?php the_title(); ?></h4></a>
<?php endforeach; ?>

Thanks for any tip..