方法一：使用 get_posts()（推荐）

<?php
if ( is_single() ) :
    global $post;
    $categories = get_the_category();
    
    foreach ($categories as $category) :
?>
<li class="widget widget_recent_entries" id="category-<?php echo $category->term_id;?>-posts">
    <h2 class="widgettitle"><?php echo $category->name; ?> 热门文章</h2>
    <ul>
        <?php
        $popular_posts = get_posts(array(
            'numberposts' => 5,
            'category' => $category->term_id,
            'meta_key' => 'views',
            'orderby' => 'meta_value_num',
            'order' => 'DESC'
        ));
        
        foreach($popular_posts as $post) :
            setup_postdata($post);
        ?>
        <li>
            <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
            <span class="view-count">(<?php echo getPostViews(get_the_ID()); ?> 阅读)</span>
        </li>
        <?php endforeach; ?>
    </ul>
</li>
<?php
    endforeach; 
    wp_reset_postdata();
endif;
?>

方法二：使用 WP_Query

<?php
if ( is_single() ) :
    global $post;
    $categories = get_the_category();
    
    foreach ($categories as $category) :
?>
<li class="widget widget_recent_entries" id="category-<?php echo $category->term_id;?>-posts">
    <h2 class="widgettitle"><?php echo $category->name; ?> 热门文章</h2>
    <ul>
        <?php
        $popular_query = new WP_Query(array(
            'posts_per_page' => 5,
            'cat' => $category->term_id,
            'meta_key' => 'views',
            'orderby' => 'meta_value_num',
            'order' => 'DESC'
        ));
        
        if ($popular_query->have_posts()) :
            while ($popular_query->have_posts()) : $popular_query->the_post();
        ?>
        <li>
            <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
            <span class="view-count">(<?php echo getPostViews(get_the_ID()); ?> 阅读)</span>
        </li>
        <?php
            endwhile;
        endif;
        wp_reset_postdata();
        ?>
    </ul>
</li>
<?php
    endforeach;
endif;
?>