wordpress中提取文章内第一张图片作为缩略图

<?php //取文章内first_image  
function catch_first_image() {  
global $post,$posts;  
$first_img = '';  
ob_start();  
ob_end_clean();  
$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i',$post->post_content,$matches);  
$first_img = $matches [1] [0];  
if(emptyempty($first_img)){  
$random = mt_rand(1,10);  
echo get_bloginfo ( 'stylesheet_directory');  
echo '/images/random/'.$random.'.jpg';//若没有则输出/images/random/内图片  
}  
return $first_img;  
}  

?>  

<?php if ( get_post_meta($post->ID, 'thumbnail', true) ) : ?><div class="thumbnail_t">  
    <?php $image = get_post_meta($post->ID, 'thumbnail', true); ?>  
    <a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title(); ?>"><img src="<?php echo $image; ?>" alt="<?php the_title(); ?>"/></a></div>  
    <?php else: ?>  

<!-- 截图 -->  
<div class="thumbnail">  
<a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title(); ?>">  
<?php if (has_post_thumbnail()) { the_post_thumbnail('full'); }  
else { ?>  
<img src="<?php echo catch_first_image() ?>" alt="<?php the_title(); ?>"/>  
<?php } ?>  
</a>  
<?php endif; ?>  
</div>  

<div id=""><?php include('includes/thumbnail.php'); ?></div>  

/*缩略图*/  
.thumbnail {  
margin-right: 10px;  
    width:140px;  
    height:100px;  
    overflow:hidden;  
    border:3px solid #e7e7e7;  
    POSITION:relative;  
float: left;  
}  
.thumbnail_t {  
margin-right: 10px;  
    width:140px;  
    height:100px;  
    overflow:hidden;  
    border:3px solid #e7e7e7;  
    POSITION:relative;  
float: left;  
}  

<?php echo mb_strimwidth(strip_tags(apply_filters('the_content',$post->post_content)),0,420,'......'); ?>  
<a title="<?php the_title(); ?>" href="<?php the_permalink() ?>">阅读全文 » </a>  

<?php the_content(__('阅读剩余部分 »')); ?>