function get_related_tag_posts_ids( $post_id, $number = 5 ) {

	$related_ids = false;

	$post_ids = array();
	// get tag ids belonging to $post_id
	$tag_ids = wp_get_post_tags( $post_id, array( 'fields' => 'ids' ) );
	if ( $tag_ids ) {
		// get all posts that have the same tags
		$tag_posts = get_posts(
			array(
				'posts_per_page' => -1, // return all posts
				'no_found_rows'  => true, // no need for pagination
				'fields'         => 'ids', // only return ids
				'post__not_in'   => array( $post_id ), // exclude $post_id from results
				'tax_query'      => array(
					array(
						'taxonomy' => 'post_tag',
						'field'    => 'id',
						'terms'    => $tag_ids,
						'operator' => 'IN'
					)
				)
			)
		);

		// loop through posts with the same tags
		if ( $tag_posts ) {
			$score = array();
			$i = 0;
			foreach ( $tag_posts as $tag_post ) {
				// get tags for related post
				$terms = wp_get_post_tags( $tag_post, array( 'fields' => 'ids' ) );
				$total_score = 0;

				foreach ( $terms as $term ) {
					if ( in_array( $term, $tag_ids ) ) {
						++$total_score;
					}
				}

				if ( $total_score > 0 ) {
					$score[$i]['ID'] = $tag_post;
					// add number $i for sorting
					$score[$i]['score'] = array( $total_score, $i );
				}
				++$i;
			}

			// sort the related posts from high score to low score
			uasort( $score, 'sort_tag_score' );
			// get sorted related post ids
			$related_ids = wp_list_pluck( $score, 'ID' );
			// limit ids
			$related_ids = array_slice( $related_ids, 0, (int) $number );
		}
	}
	return $related_ids;
}


function sort_tag_score( $item1, $item2 ) {
	if ( $item1['score'][0] != $item2['score'][0] ) {
		return $item1['score'][0] < $item2['score'][0] ? 1 : -1;
	} else {
		return $item1['score'][1] < $item2['score'][1] ? -1 : 1; // ASC
	}
}