Add Open Graph Markup to WordPress Theme

functions.php

<?php
function add_opengraph_markup() {
  if (is_single()) {
    global $post;
    if(get_the_post_thumbnail($post->ID, 'thumbnail')) {
      $thumbnail_id = get_post_thumbnail_id($post->ID);
      $thumbnail_object = get_post($thumbnail_id);
      $image = $thumbnail_object->guid;
    } else {
      // set default image
      $image = '';
    }
    //$description = get_bloginfo('description');
    $description = substr(strip_tags($post->post_content),0,200) . '...';
?>
<meta property="og:title" content="<?php the_title(); ?>" />
<meta property="og:type" content="article" />
<meta property="og:image" content="<?=$image?>" />
<meta property="og:url" content="<?php the_permalink(); ?>" />
<meta property="og:description" content="<?=$description?>" />
<meta property="og:site_name" content="<?=get_bloginfo('name')?>" />

<?php
  }
}
add_action('wp_head', 'add_opengraph_markup');
?>

If no image is set for a post or page, the $image variable can set or dynamically constructed to point to an image on your server. To point it to a static file, line 11 would look like:

$image = 'http://yourserver.com/images/whatever.png';

added this to my theme's function.php file, now wp-admin won't return anything but a blank page.
removing this code corrects the wp-admin problem.
any idea what's happening ?