Post Thumbnail (Featured Image) Captions in WordPress

has_post_thumbnail_caption.php

/**
 * Check if Post Thumbnail (Featured Image) has a caption, return true if it does
 */
function has_post_thumbnail_caption() {
  global $post;

  $thumbnail_id    = get_post_thumbnail_id($post->ID);
  $thumbnail_image = get_posts(array('p' => $thumbnail_id, 'post_type' => 'attachment'));
  $thumbnail_excerpt = $thumbnail_image[0]->post_excerpt;

  if ($thumbnail_excerpt) {
    return true;
  } else {
    return false;
  }
}

the_post_thumbail_caption.php

/**
 * Display Post Thumbnail (Featured Image) Caption
 */
function the_post_thumbnail_caption() {
  global $post;

  $thumbnail_id    = get_post_thumbnail_id($post->ID);
  $thumbnail_image = get_posts(array('p' => $thumbnail_id, 'post_type' => 'attachment'));

  if ($thumbnail_image && isset($thumbnail_image[0])) {
    echo '<span>'.$thumbnail_image[0]->post_excerpt.'</span>';
  }
}