Open Graph protocol for WordPress

header_ogp.php

<?php
/**
 * Open Graph protocol for WordPress
 * @version 0.9.2
 * @author makoto_kw
 * @link https://gist.github.com/3399585
 */
// key into custom fields for description. Default is for All in One SEO Pack
define('WP_OGP_POST_DESCRIPTION_KEY', '_aioseop_description');

// size of Feature Image
// http://codex.wordpress.org/Post_Thumbnails
define('WP_OGP_POST_IMAGE_SIZE', 'medium');

// default Image Url
define('WP_OGP_DEFAULT_IMG', bloginfo('template_url').'/images/ogp_default.jpg');

// locale
define('WP_OGP_LOCALE', 'ja_JP');

// Facebook AppId and username
// http://developers.facebook.com/docs/opengraphprotocol/
//define('WP_OGP_FB_APPID', 'your-appid');
//define('WP_OGP_FB_ADMINS', 'your-admins');

// Twitter Usernames
//define('WP_OGP_TWITTER_SITE_USERNAME', '@site_username');
//define('WP_OGP_TWITTER_CREATOR_USERNAME', '@creator_username');
//define('WP_OGP_TWITTER_DOMAIN', 'YourDomain.com');

function ogp_post_description() {
  global $post;
  $description = null;
  if (defined('WP_OGP_POST_DESCRIPTION_KEY')) {
    $description = get_post_meta($post->ID, WP_OGP_POST_DESCRIPTION_KEY, true);
  }
  if (empty($description)) {
    $description = get_the_excerpt();
  }
  return $description;
}
function ogp_post_image() {
  $image = null;
  if ($image_id = get_post_thumbnail_id()) {
    $image = wp_get_attachment_image_src($image_id, WP_OGP_POST_IMAGE_SIZE);
  }
  if (empty($image)) {
    // find first img element
    global $post;
    if (preg_match('/<img[^>]*src\s*=\s*("|\')([^"\']+)("|\')[^>]*>/i', $post->post_content, $matches)) {
      return $matches[2];
    }
  }
  return $image;
}
function ogp_post_section() {
  $categories = get_the_category();
  if (count($categories)>0) {
    if ($categories[0]->name !=  __('Uncategorized')) {
      return $categories[0]->name;
    } 
  }
  return false;
}
function ogp_post_tag() {
  $tags = get_the_tags();
  $tagnames = array();
  if ($tags) {
    foreach ($tags as $tag) {
      $tagnames[] = $tag->name;
    }
    return implode(',', $tagnames);
  }
  return false;
}
?>
<?php if( is_single() || is_page()): ?>
<?php while(have_posts()): the_post(); ?>
<meta property="og:title" content="<?php the_title();?>" />
<meta property="og:type" content="article" />
<?php if ($og_image = ogp_post_image()):?>
<meta property="og:image" content="<?php echo $og_image ?>" />
<?php else:?>
<meta property="og:image" content="<?php echo WP_OGP_DEFAULT_IMG ?>" />
<?php endif ?>
<meta property="og:url" content="<?php echo esc_url(get_permalink()); ?>" />
<?php if ($og_description = ogp_post_description()):?>
<meta property="og:description" content="<?php echo esc_attr($og_description) ?>" />
<?php else:?>
<meta property="og:description" content="<?php bloginfo('description'); ?>" />
<?php endif ?>
<!-- <meta property="article:author" content="<?php echo get_the_author(); ?>" /> -->
<meta property="article:published_time" content="<?php echo get_post_time('c') ?>" />
<meta property="article:modified_time" content="<?php echo get_the_modified_time('c') ?>" />
<?php if ($og_section = ogp_post_section()):?>
<meta property="article:section" content="<?php echo esc_attr($og_section) ?>" />
<?php endif ?>
<?php if ($og_tag = ogp_post_tag()):?>
<meta property="article:tag" content="<?php echo esc_attr($og_tag) ?>" />
<?php endif ?>
<meta property="twitter:card" content="summary" />
<meta property="twitter:title" content="<?php echo mb_strimwidth(get_the_title(), 0, 70, '...'); ?>" />
<?php if ($og_description):?>
<meta property="twitter:description" content="<?php echo mb_strimwidth(esc_attr($og_description), 0, 200); ?>" />
<?php endif; ?>
<?php if ($og_image):?>
<meta property="twitter:image:src" content="<?php echo $og_image ?>" />
<?php else: ?>
<meta property="twitter:image:src" content="<?php echo WP_OGP_DEFAULT_IMG ?>" />
<?php endif; ?>
<?php if (defined('WP_OGP_TWITTER_SITE_USERNAME')): ?>
<meta property="twitter:site" content="<?php echo WP_OGP_TWITTER_SITE_USERNAME; ?>" />
<?php endif ?>
<?php if (defined('WP_OGP_TWITTER_CREATOR_USERNAME')): ?>
<meta property="twitter:creator" content="<?php echo WP_OGP_TWITTER_CREATOR_USERNAME; ?>" />
<?php endif ?>
<?php if (defined('WP_OGP_TWITTER_DOMAIN')): ?>
<meta property="twitter:domain" content="<?php echo WP_OGP_TWITTER_DOMAIN; ?>" />
<?php endif ?>
<?php endwhile; ?>
<?php else: ?>
<meta property="og:title" content="<?php wp_title('|', true, 'right'); ?>" />
<?php if (is_front_page() || is_404()): ?>
<meta property="og:type" content="blog" />
<meta property="og:url" content="<?php echo get_bloginfo('url')?>" />
<?php else: ?>
<meta property="og:type" content="article" />
<meta property="og:url" content="<?php echo get_pagenum_link()?>" />
<?php endif ?>
<meta property="og:image" content="<?php echo WP_OGP_DEFAULT_IMG?>" />
<meta property="og:description" content="<?php bloginfo('description'); ?>" />
<?php endif ?>
<meta property="og:locale" content="<?php echo WP_OGP_LOCALE?>" />
<meta property="og:site_name" content="<?php bloginfo('name'); ?>" />
<?php if (defined('WP_OGP_FB_APPID')): ?>
<meta property="fb:app_id" content="<?php echo WP_OGP_FB_APPID; ?>" />
<?php endif ?>
<?php if (defined('WP_OGP_FB_ADMINS')): ?>
<meta property="fb:admins" content="<?php echo WP_OGP_FB_ADMINS; ?>" />
<?php endif ?>

Installation

1. copy this file to your selected theme directory. (ex wp-content/theme/twentytwelve/)
2. modify header.php (ex: using TwentyTwelve )

<html <?php language_attributes(); ?>>
<head prefix="og: http://ogp.me/ns# fb: http://ogp.me/ns/fb# article: http://ogp.me/ns/article#"> <!-- modified -->
<meta charset="<?php bloginfo( 'charset' ); ?>" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title><?php wp_title( '|', true, 'right' ); ?></title>
<?php include('header_ogp.php');?> <!-- added-->
<link rel="pingback" href="<?php bloginfo( 'pingback_url' ); ?>" />