Missing description in FB Opengraph

Hi,

I’m editing Functions.php to generate FB Opengraph metatags, everything right till now but I can’t get the post description.

My code is:

function fb_opengraph() {
    global $post;

    if(is_single()) {
        if(has_post_thumbnail($post->ID)) {
            $img_src = wp_get_attachment_image_src( get_post_thumbnail_id( get_the_ID() ), ‘medium’);
$img_src = $img_src[0];
        } else {
            $img_src = get_stylesheet_directory_uri() . '/img/opengraph_image.jpg';
        }
        if($excerpt = $post->post_excerpt) {
            $excerpt = strip_tags($post->post_excerpt);
            $excerpt = str_replace("", "'", $excerpt);
        } else {
            $excerpt = get_bloginfo('description');
        }
        ?>

I’m obtaining an empty string in $excerpt

Any help would be appreciated.

Cheers