Wordpress related posts

related-posts.php

$related = get_posts( 
    array( 
        'category__in' => wp_get_post_categories( $post->ID ), 
        'numberposts'  => 5, 
        'post__not_in' => array( $post->ID ) 
    ) 
);

if( $related ) { 
    foreach( $related as $post ) {
        setup_postdata($post);
        /*whatever you want to output*/
    }
    wp_reset_postdata();
}

////////////////////////
Reference:
- get_posts
- wp_reset_postdata
- wp_get_post_categories
////////////////////////

Answer re-written based on WP_Query():

$related = new WP_Query(
    array(
        'category__in'   => wp_get_post_categories( $post->ID ),
        'posts_per_page' => 5,
        'post__not_in'   => array( $post->ID )
    )
);

if( $related->have_posts() ) { 
    while( $related->have_posts() ) { 
        $related->the_post(); 
        /*whatever you want to output*/
    }
    wp_reset_postdata();
}



// for functions.php

function example_cats_related_post() {

    $post_id = get_the_ID();
    $cat_ids = array();
    $categories = get_the_category( $post_id );

    if(!empty($categories) && is_wp_error($categories)):
        foreach ($categories as $category):
            array_push($cat_ids, $category->term_id);
        endforeach;
    endif;

    $current_post_type = get_post_type($post_id);
    $query_args = array( 

        'category__in'   => $cat_ids,
        'post_type'      => $current_post_type,
        'post_not_in'    => array($post_id),
        'posts_per_page'  => '3'


     );

    $related_cats_post = new WP_Query( $query_args );

    if($related_cats_post->have_posts()):
         while($related_cats_post->have_posts()): $related_cats_post->the_post(); ?>
            <ul>
                <li>
                    <a href="<?php the_permalink(); ?>">
                        <?php the_title(); ?>
                    </a>
                    <?php the_content(); ?>
                </li>
            </ul>
        <?php endwhile;

        // Restore original Post Data
        wp_reset_postdata();
     endif;

}

Now you can simply call the function anywhere in your site using:
php example_cats_related_post()
  
reference: https://wordpress.stackexchange.com/questions/30039/how-to-display-related-posts-from-same-category