multibyte custom excerpt code

I found this code on-line as I am trying to return 140 characters for multibyte Japanese characters on a custom front page, but I am doing something wrong as it is not returning anything. I am not sure if is because strip_tags is not a defined function. Can someone please tell me what I need to get this to work? Thank you.

front.php –

<?php
    $recentPosts = new WP_Query();
    $recentPosts->query('showposts=5');
?>
<?php while ($recentPosts->have_posts()) : $recentPosts->the_post(); ?>

<li><a>" rel="bookmark"><?php echo winexcerpt(140); ?></a></li>
<?php endwhile; ?>

functions.php –

function winexcerpt( $length ) {
   global $post;
   $content = mb_substr(strip_tags($post->post_content),0,$length);
   return $content;
}