Related Posts Without Plugins

<?php
/**
 * Outputs related posts with thumbnail.
 * This function is based on https://netmospherics.com/blog/wordpress-related-posts-without-plugin/
 * Require this file inside theme functions.php // require_once 'inc/helpers/single-related-posts.php';
 * To Be used inside single post before end of while loop // showRelatedPosts('post', get_the_ID(), 4, 'tags');
 * @param  string $postType
 * @param  integer $postID
 * @param  integer $totalPosts Number of related posts to display
 * @param  string $relatedBy Specify posts related by 'tags' || 'category'
 * @author Waseem Mansour <waseem.mn2002@gmail.com>
 */

function showRelatedPosts($postType = 'post', $postID = null, $totalPosts = null, $relatedBy = null) {
	global $post, $related_posts_custom_query_args;
	if (null === $postID) $postID = $post->ID;
	if (null === $totalPosts) $totalPosts = 4;
	if (null === $relatedBy) $relatedBy = 'category';
	if (null === $postType) $postType = 'post';

	// Build our basic custom query arguments

	if ($relatedBy === 'category') {
		$categories = get_the_category( $post->ID );
		$catidlist = '';
		foreach( $categories as $category) {
			$catidlist .= $category->cat_ID . ",";
		}
		// Build our category based custom query arguments
		$related_posts_custom_query_args = array(
			'post_type' => $postType,
			'posts_per_page' => $totalPosts, // Number of related posts to display
			'post__not_in' => array($postID), // Ensure that the current post is not displayed
			'orderby' => 'rand', // Randomize the results
			'cat' => $catidlist, // Select posts in the same categories as the current post
		);
	}

	if ($relatedBy === 'tags') {

		// Get the tags for the current post
		$tags = wp_get_post_tags($postID);
		// If the post has tags, run the related post tag query
		if ($tags) {
			$tag_ids = array();
			foreach($tags as $individual_tag) $tag_ids[] = $individual_tag->term_id;
			// Build our tag related custom query arguments
			$related_posts_custom_query_args = array(
				'post_type' => $postType,
				'tag__in' => $tag_ids, // Select posts with related tags
				'posts_per_page' => $totalPosts, // Number of related posts to display
				'post__not_in' => array($postID), // Ensure that the current post is not displayed
				'orderby' => 'rand', // Randomize the results
			);
		} else {
			// If the post does not have tags, run the standard related posts query
			$related_posts_custom_query_args = array(
				'post_type' => $postType,
				'posts_per_page' => $totalPosts, // Number of related posts to display
				'post__not_in' => array($postID), // Ensure that the current post is not displayed
				'orderby' => 'rand', // Randomize the results
			);
		}

	}

	// Initiate the custom query
	$custom_query = new WP_Query( $related_posts_custom_query_args );


	// Run the loop and output data for the results
	if ( $custom_query->have_posts() ) : ?>
		<?php while ( $custom_query->have_posts() ) : $custom_query->the_post(); ?>

			<article class="col-sm-4">
				<div class="post-thumbnail">
					<a href="https://www.reviano.com/blog/top-5-benefits-of-time-tracking-software.html">
						<img src="<?php the_post_thumbnail_url( 'card-thumb' ) ?>" alt="<?php the_title() ?>" name="<?php the_title(); ?>">
					</a>
				</div>
				<div class="entry-desc">
					<span class="h3">
						<a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
					</span>
				</div>
			</article>

		<?php endwhile; ?>
	<?php else : ?>
		<p>Sorry, no related articles to display.</p>
	<?php endif;
	// Reset postdata
	wp_reset_postdata();
}

https://gist.github.com/WaseemMansour/f9bbbe558e935e7c06f93ee398074221

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