Related Post Category

related_post_category.php

<?php 
/**
 * Return html
 */
function related_post() {

    $post_id = get_the_ID();
    $post_type = get_post_type($post_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;

    $args = array( 
        'post_type'      => $post_type,
        'category__in'   => $cat_ids,
        'post__not_in'    => array($post_id),
        'posts_per_page'  => '6'
     );

    $query = new WP_Query( $args );

    if($query->have_posts()): ?>

        <ul>

         <?php while($query->have_posts()): $query->the_post(); ?>

                <li>

                    <a href="<?php the_permalink(); ?>">

                        <?php the_title(); ?>

                    </a>

                    <?php the_content(); ?>

                </li>

        <?php endwhile; ?>

        </ul>

        wp_reset_postdata();

     endif;
}
?>

<?php echo related_post(); ?>