<h3>相关文章</h3>
<ul class="related_posts">
<?php
$post_num = 8;
$exclude_id = $post->ID;
$posttags = get_the_tags(); $i = 0;
if ( $posttags ) {
    $tags = ''; foreach ( $posttags as $tag ) $tags .= $tag->term_id . ',';
    $args = array(
        'post_status' => 'publish',
        'tag__in' => explode(',', $tags),
        'post__not_in' => explode(',', $exclude_id),
        'caller_get_posts' => 1,
        'orderby' => 'comment_date',
        'posts_per_page' => $post_num,
    );
    query_posts($args);
    while( have_posts() ) { the_post(); ?>
        <li><a rel="bookmark" href="<?php%20the_permalink();%20?>" title="<?php the_title(); ?>" target="_blank"><?php the_title(); ?></a></li>
    <?php
        $exclude_id .= ',' . $post->ID; $i ++;
    } wp_reset_query();
}
if ( $i < $post_num ) {
    $cats = ''; foreach ( get_the_category() as $cat ) $cats .= $cat->cat_ID . ',';
    $args = array(
        'category__in' => explode(',', $cats),
        'post__not_in' => explode(',', $exclude_id),
        'caller_get_posts' => 1,
        'orderby' => 'comment_date',
        'posts_per_page' => $post_num - $i
    );
    query_posts($args);
    while( have_posts() ) { the_post(); ?>
        <li><a rel="bookmark" href="<?php%20the_permalink();%20?>"  title="<?php the_title(); ?>" target="_blank"><?php the_title(); ?></a></li>
    <?php $i++;
    } wp_reset_query();
}
if ( $i  == 0 )  echo '<li>没有相关文章!</li>';
?>
</ul>

注意：第4行$post_num=8；表示显示了8篇文章，请根据您的需要修改它们。

显示样式需要编写自己的CSS，以便参考以下内容：

.related_posts{margin-top:5px;}
.related_posts li{margin-left:20px;color:#444;list-style:circle;font-size:14px;line-height:26px;padding:0 0 0 5px}

2.添加含缩略图的相关文章

WordPress 添加含缩略图的相关文章

找到 functions.php 后，在该文件最后一个 ?> 前添加下面的代码：

//添加特色缩略图支持
if ( function_exists('add_theme_support') )add_theme_support('post-thumbnails');
 
//输出缩略图地址 From wpdaxue.com
function post_thumbnail_src(){
    global $post;
	if( $values = get_post_custom_values("thumb") ) {	//输出自定义域图片地址
		$values = get_post_custom_values("thumb");
		$post_thumbnail_src = $values [0];
	} elseif( has_post_thumbnail() ){    //如果有特色缩略图，则输出缩略图地址
        $thumbnail_src = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID),'full');
		$post_thumbnail_src = $thumbnail_src [0];
    } else {
		$post_thumbnail_src = '';
		ob_start();
		ob_end_clean();
		$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
		$post_thumbnail_src = $matches [1] [0];   //获取该图片 src
		if(empty($post_thumbnail_src)){	//如果日志中没有图片，则显示随机图片
			$random = mt_rand(1, 10);
			echo get_bloginfo('template_url');
			echo '/images/pic/'.$random.'.jpg';
			//如果日志中没有图片，则显示默认图片
			//echo '/images/default_thumb.jpg';
		}
	};
	echo $post_thumbnail_src;
}

注意：以上代码主要是获取图片链接，获取顺序为：

自定义字段为 thumb 的图片>特色缩略图>文章第一张图片>随机图片/默认图片；；

随机图片：请制作10张图片，放到当前主题文件夹下的images/PIC/目录中，图片为JPG格式，使用数字1-10命名，如1.JPG；如果您不想使用随机图片，请删除最后5行前的“/”，然后在最后7行和9行中添加“/”以注销，并在当前主题的/images/目录下添加默认名称拇指.jpg默认图像，以便显示默认图像。

把一下代码代码复制到 single.php 中要显示相关文章的位置即可：

<h3>相关文章</h3>
<ul class="related_img">
<?php
$post_num = 4;
$exclude_id = $post->ID;
$posttags = get_the_tags(); $i = 0;
if ( $posttags ) {
	$tags = ''; foreach ( $posttags as $tag ) $tags .= $tag->term_id . ',';
	$args = array(
		'post_status' => 'publish',
		'tag__in' => explode(',', $tags),
		'post__not_in' => explode(',', $exclude_id),
		'caller_get_posts' => 1,
		'orderby' => 'comment_date',
		'posts_per_page' => $post_num
	);
	query_posts($args);
	while( have_posts() ) { the_post(); ?>
		<li class="related_box"  >
		<div class="r_pic">
		<a href="<?php%20the_permalink();%20?>" title="<?php the_title(); ?>" target="_blank">
		<img src="<?php%20echo%20post_thumbnail_src();%20?>" alt="<?php the_title(); ?>" class="thumbnail" />
		</a>
		</div>
		<div class="r_title"><a href="<?php%20the_permalink();%20?>" title="<?php the_title(); ?>" target="_blank" rel="bookmark"><?php the_title(); ?></a></div>
		</li>
	<?php
		$exclude_id .= ',' . $post->ID; $i ++;
	} wp_reset_query();
}
if ( $i < $post_num ) {
	$cats = ''; foreach ( get_the_category() as $cat ) $cats .= $cat->cat_ID . ',';
	$args = array(
		'category__in' => explode(',', $cats),
		'post__not_in' => explode(',', $exclude_id),
		'caller_get_posts' => 1,
		'orderby' => 'comment_date',
		'posts_per_page' => $post_num - $i
	);
	query_posts($args);
	while( have_posts() ) { the_post(); ?>
	<li class="related_box"  >
		<div class="r_pic">
		<a href="<?php%20the_permalink();%20?>" title="<?php the_title(); ?>" target="_blank">
		<img src="<?php%20echo%20post_thumbnail_src();%20?>" alt="<?php the_title(); ?>" class="thumbnail" />
		</a>
		</div>
		<div class="r_title"><a href="<?php%20the_permalink();%20?>" title="<?php the_title(); ?>" target="_blank" rel="bookmark"><?php the_title(); ?></a></div>
	</li>
	<?php $i++;
	} wp_reset_query();
}
if ( $i  == 0 )  echo '<div class="r_title">没有相关文章!</div>';
?>
</ul>

注意：在代码的第四行$post_num = 4; 表示要显示4篇文章，请根据自己需要修改。

CSS参考:

.related_posts{margin-top:5px;}
.related_img{width:600px;height:210px;}
.related_box{float:left;overflow:hidden;margin-top:5px;width:148px;border-right:1px #eee solid}
.related_box:hover{background:#f9f9f9}
.related_box .r_title{width:auto;height:72px;font-weight:400;font-size:14px;margin:0 10px;overflow:hidden;}
.related_box .r_pic{margin:6px}
.related_box .r_pic img{width:130px;height:100px;border:1px  solid #e1e1e1;background:#fff;padding:2px}