This script is for showing related post in WordPress.

related_post.php

/*
*
*
* Get Related Posts
*function to get related post having codition inside
*
*/
function get_related_post($postID){
$themeTemplateDirectoryUri = get_template_directory_uri();
  ?>

  <div class="row">
  <?php

			$tags = wp_get_post_tags($postID);
			$foundPosts = 0;
			$excludes=[$postID];

			if ($tags) {
					$tag_ids = array();
					foreach($tags as $individual_tag) $tag_ids[] = $individual_tag->term_id;

					$args=array(
							'tag__in' => $tag_ids,
							'post__not_in' => array($postID),
							'showposts'=> 3,
							'ignore_sticky_posts'=>1
						 );

				$my_query = new WP_Query($args);
				$foundPosts = $my_query->found_posts;

				if( $my_query->have_posts() ) {
					while ($my_query->have_posts()) : $my_query->the_post();
					$excludes[]=$my_query->post->ID;
          $thumbImage = wp_get_attachment_url(get_post_thumbnail_id($my_query->post->ID) );
          $image = ( $thumbImage && $thumbImage != '' ) ? $thumbImage : $themeTemplateDirectoryUri.'/img/product-img6.jpg';
           ?>
           <div class="col-sm-4">
             <div class="related-post-content area">
                  <a href="<?php the_permalink(); ?>"><img src="<?php echo $image; ?>" alt="" class="img-responsive"></a>
               <a href="<?php the_permalink(); ?>"><h4 class="font-bold"><?php the_title(); ?></h4></a>
               <?php the_excerpt(); ?>
             </div>
           </div>
						<?php
					endwhile;
				}
				wp_reset_query();
			}

			if ($foundPosts <3){
					$remainingPosts = 3 - $foundPosts;
					get_category_posts($postID, $remainingPosts, $excludes);
				}
?>
</div>
<?php
}

//Get remaining post from category by that given post id
function get_category_posts($postID, $remainingPosts, $excludes){

  $themeTemplateDirectoryUri = get_template_directory_uri();
				$catArgs = array(
												'category__in' =>  wp_get_post_categories($postID),
												'showposts' => $remainingPosts,
												'post__not_in' => $excludes
										);

		 		$cat_post_query = new WP_Query($catArgs);
				$foundPosts = $cat_post_query->found_posts;

				if( $cat_post_query->have_posts() ) {

					while ($cat_post_query->have_posts()) : $cat_post_query->the_post();
						$excludes[]=$cat_post_query->post->ID;
            $thumbImage = wp_get_attachment_url(get_post_thumbnail_id($cat_post_query->post->ID) );
            $image = ( $thumbImage && $thumbImage != '' ) ? $thumbImage : $themeTemplateDirectoryUri.'/img/product-img6.jpg';
             ?>

             <div class="col-sm-4">
               <div class="related-post-content area">
                 <a href="<?php the_permalink(); ?>"><img src="<?php echo $image; ?>" alt="" class="img-responsive"></a>
                 <a href="<?php the_permalink(); ?>"><h4 class="font-bold"><?php the_title(); ?></h4></a>
                 <?php the_excerpt(); ?>
               </div>
             </div>

						<?php
					endwhile;
				}
				wp_reset_query();



     	if ($foundPosts < $remainingPosts){
				$stillRemaing = $remainingPosts-$foundPosts;

			get_latest_posts($excludes, $stillRemaing);

    }
  }


//Get rest of the posts from latest post types

function get_latest_posts($excludes, $stillRemaing){


			$latestPost = array(
					'showposts' => $stillRemaing,
					'post__not_in' => $excludes,
					'post_type' => 'post',
					'posts_per_page' => $stillRemaing,
					'offset'=>0,
					'orderby' => 'post_date',
					'order' => 'DESC',
			);
			$latest_post_query = new WP_Query($latestPost);

			if( $latest_post_query->have_posts() ) {
					while ($latest_post_query->have_posts()) : $latest_post_query->the_post();
          $thumbImage = wp_get_attachment_url(get_post_thumbnail_id($latest_post_query->post->ID) );
           ?>
           <div class="col-sm-4">
             <div class="related-post-content area">
               <a href="<?php the_permalink(); ?>"><img src="<?php echo $image; ?>" alt="" class="img-responsive"></a>
               <a href="<?php the_permalink(); ?>"><h4 class="font-bold"><?php the_title(); ?></h4></a>
               <?php the_excerpt(); ?>
             </div>
           </div>
					<?php
					endwhile;
				}
				wp_reset_query();
	}

//echo get_related_post(get_the_id());
//echo get_related_post($post->ID);