gistfile1.txt

function rp4wp_force_same_category( $sql, $post_id, $post_type ) {
	global $wpdb;

	if ( 'post' !== $post_type ) {
		return $sql;
	}

	$sql_replace = "
	INNER JOIN " . $wpdb->term_relationships . " ON (R.`post_id` = " . $wpdb->term_relationships . ".object_id)
	INNER JOIN " . $wpdb->term_taxonomy . " ON (" . $wpdb->term_relationships . ".term_taxonomy_id = " . $wpdb->term_taxonomy . ".term_taxonomy_id)
	WHERE 1=1
	AND " . $wpdb->term_taxonomy . ".taxonomy = 'post_tag'
	AND " . $wpdb->term_taxonomy . ".term_id IN ( SELECT TT.term_id FROM " . $wpdb->term_taxonomy . " TT INNER JOIN " . $wpdb->term_relationships . " TR ON TR.term_taxonomy_id = TT.term_taxonomy_id WHERE TR.object_id = " . $post_id . " )
	";

	return str_ireplace( 'WHERE 1=1', $sql_replace, $sql );
}

add_filter( 'rp4wp_get_related_posts_sql', 'rp4wp_force_same_category', 11, 3 );