Getting related articles in WordPress not working - php

Question

I am trying to display all the posts with the same category in WordPress however its not displaying correctly and instead is just showing everything.
Here is the php code:
<?php
$related = get_posts( array(
'category_in' => wp_get_post_categories($post->ID),
'numberposts' => 3,
'post_not_in' => array($post->ID) ) );
if( $related ) foreach( $related as $post ) {
setup_postdata($post); ?>
<div class="post">
<a href="<?php the_permalink(); ?>">
<?php if ( has_post_thumbnail() ) {
the_post_thumbnail();
}
?>
<h3><?php the_title();?></h3>
</a>
</div>
<?php }
wp_reset_postdata();
?>
Its taken from here: https://wordpress.stackexchange.com/questions/41272/how-to-show-related-posts-by-category
And if it helps here is the link to the website in question where the code isnt working:http://u1f8aki.nixweb23.dandomain.dk/cat-4-post-test/
The code in question is further down the page under the red text. You can see the category at the top in the breadcrumbs.

Answer 1

There are quite errors in your code, and I'm sure atleast one of them are the culprit in giving you the wrong result.
I've refactored your code with some comments explaining what and why has been changed:
<?php
// For readability, save our categories in a variable for later use.
// $post->ID has been replaced with get_the_ID(), $post might not be accessible depending if you're exposing $post as a global or not.
$categories = wp_get_post_categories(get_the_ID());
/*
Instead of using get_posts(), use the recommended Wordpress loop in the form of WP_Query().
We start by defining our arguments for the loop
*/
$args = array(
'category_in' => $categories, // here we use variable for readability
'posts_per_page' => 3, //numberposts and posts_per_page has the same function, but posts_per_page is the more common of the two (IMO)
'post__not_in' => array(get_the_ID()) // you were missing a '_', ie post_not_in instead of post__not_in
);
// Start the loop
$query = new WP_Query($args);
if($query->have_posts()): while($query->have_posts()): $query->the_post();
// No need to setup or reset postdata when using this method, it does it for you!
?>
<div class="post">
<a href="<?php the_permalink(); ?>">
<?php
if( has_post_thumbnail() ) {
the_post_thumbnail();
}
?>
<?php
// the_title() actually takes opening tag and closing tags as arguments in its function. So add the <h3> code like this.
the_title('<h3>', '</h3>');
?>
</a>
</div>
<?php endwhile; endif; ?>
Long story short, your code prolly isn't working because of the miss-named arguments. If you don't feel like replacing your code with my example, just change your arguments from numberposts to posts_per_page, and post_not_in to post__not_in.
If it still isn't working, check what wp_get_post_categories(get_the_ID()) is returning for each post, and make sure all posts aren't sharing some category you missed.
Edit: numberposts is actually a valid argument, changed my answer to reflect this.