<?php
/*
single page?show current category articles
*/
?>
<?php
if ( is_single() ) :
global $post;
$categories = get_the_category();
foreach ($categories as $category) :
    ?>
    <li class="widget widget_recent_entries" id="<?php $category->term_id;?>-posts">
        <h2 class="widgettitle"><?php echo $category->name; ?></h2>
        <ul>
        <?php
        $posts = get_posts('numberposts=5&category='. $category->term_id);
        foreach($posts as $post) :
        ?>
            <li>
            <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
            </li>
        <?php endforeach; ?>
        </ul>
    </li>
<?php
endforeach; endif ; ?>
<?php
/*
end show current category articles
*/
?>