//相关文章
function add_related_posts($content){
    return $content . wp_related_posts();
}
add_filter ('the_content', 'add_related_posts'); //hook
function wp_related_posts(){
    global $post;
    $num = 5;//文章数量
    $counter = 1;
    $exclude_id = get_post_meta($post->ID,'related',true);//获取手动置顶的相关文章
    if ($exclude_id){
        $args = array(
            'post_status' => 'publish',
            'post_type' => array('post'),
            'post__in' => explode(',', $exclude_id),
            'posts_per_page' => $num
        );
        $posts = get_posts($args);
        foreach($posts as $sb){
            $output .= '<li><a href="'%20.%20get_permalink($sb->ID)%20.%20'">' . $sb->post_title . '</a></li>';//可自定义样式
            $i++;
        }
    }
    if( $i < $num){//自定义文章数不足后通过分类和标签处理
        $tagsid = array();
        $catid = array();
        $thisid[] = $post->ID;
        $posttags = get_the_tags();
        $catids = get_the_category();
        if(!empty($posttags)) {
            foreach($posttags as $tag) {
                $tagsid[] = $tag->term_id;
            }
        }
        if(!empty($catids)) {
            foreach($catids as $cat) {
                $catid[] = $cat->term_id;
            }
        }
        $args = array(
            'post_type' => 'post',
            'post__not_in' => $thisid,
            'ignore_sticky_posts' => 1,
            'posts_per_page' => ($num - $i),
            'tax_query' => array(
                'relation' => 'OR',//改成AND则必须是同标签同分类下
                array(
                    'taxonomy' => 'post_tag',
                    'field'    => 'term_id',
                    'terms'    => $tagsid,
                ),
                array(
                    'taxonomy' => 'category',
                    'field'    => 'term_id',
                    'terms'    => $catid,
                ),
            ),
        );
        $rsp = get_posts($args );
        foreach($rsp as $sb){
            $output .= '<li><a href="'%20.%20get_permalink($sb->ID)%20.%20'">' . $sb->post_title . '</a></li>';//可自定义样式
            $i++;
        }
    }
    $final = '<h3>相关文章</h3><ul>' . $output . '</ul>';
    return $final;
}