related-posts-form-current-taxonomy

<?php


function pippin_related_posts($taxonomy = '') {

	global $post;

	if($taxonomy == '') { $taxonomy = 'post_tag'; }

	$tags = wp_get_post_terms($post->ID, $taxonomy);

	if ($tags) {
		$first_tag 	= $tags[0]->term_id;
		$second_tag = $tags[1]->term_id;
		$third_tag 	= $tags[2]->term_id;
		$args = array(
			'post_type' => get_post_type($post->ID),
			'posts_per_page' => 4,
			'tax_query' => array(
				'relation' => 'OR',
				array(
					'taxonomy' => $taxonomy,
					'terms' => $second_tag,
					'field' => 'id',
					'operator' => 'IN',
				),
				array(
					'taxonomy' => $taxonomy,
					'terms' => $first_tag,
					'field' => 'id',
					'operator' => 'IN',
				),
				array(
					'taxonomy' => $taxonomy,
					'terms' => $third_tag,
					'field' => 'id',
					'operator' => 'IN',
				)
			)
		);
		$related = get_posts($args);
		$i = 0;
		if( $related ) {
			global $post;
			$temp_post = $post;
				foreach($related as $post) : setup_postdata($post);
					$content .= '<ul class="related-posts-box">';
						$content .= '<li><a href="' . get_permalink() . '" title="' . get_the_title() . '">' . get_the_title() . '</a></li>';
					$content .= '</ul>';

				endforeach;
			$post = $temp_post;
		}
	}

	return $content;
}


add_action('the_content', 'do_jt_related_posts');
function do_jt_related_posts() {


    if( is_singular('post') ) :
        echo get_the_content();
        echo pippin_related_posts();
    else :
        echo get_the_content();
    endif;
}