plugin title attr post thumbnail

<?php
/*
Plugin Name: Featured Image Title Attribute Output
Plugin URI: n/a
Description: adds the image title as the title attribute to the 'featured image' output.
Version: 1.0
Author: alchymyth
Author URI: http://forum.wpde.org/mitglied/23424-alchymyth.html
License: GPL2
*/

add_filter('post_thumbnail_html','add_post_thumbnail_title_attribute',10,3);

function add_post_thumbnail_title_attribute($html, $post_id, $post_thumbnail_id) {

if( $html == '' ) {
    return $html;
} else {
    $out = $html;
  $thumbnail_image = get_posts(array('p' => $post_thumbnail_id, 'post_type' => 'attachment'));

  if ($thumbnail_image && isset($thumbnail_image[0])) {

    $title = $thumbnail_image[0]->post_title;
    if($title) {
        $output = ' title="' . esc_attr($title) . '" ';
        $out = str_replace( 'class=', $output . 'class=', $html );
     }
  }
}
return $out;
}
?>