Add Open Graph tags to Wordpress

functions.php

function doctype_opengraph($output) {
    return $output . '
    xmlns:og="http://opengraphprotocol.org/schema/"
    xmlns:fb="http://www.facebook.com/2008/fbml"';
}
add_filter('language_attributes', 'doctype_opengraph');

function fb_opengraph() {
    global $post;

    if(has_post_thumbnail($post->ID)) {
        $img_src = wp_get_attachment_image_src(get_post_thumbnail_id( $post->ID ), 'medium');
    } else {
        $img_src = get_stylesheet_directory_uri() . '/img/opengraph_image.jpg';
    }

    if($post->post_excerpt != '') {
        $excerpt = strip_tags($post->post_excerpt);
        $excerpt = str_replace("", "'", $excerpt);
    } elseif ($post->post_content != ''){
        $excerpt = strip_tags($post->post_content);
        $excerpt = str_replace("", "'", $excerpt);
        $excerpt = wp_trim_words($excerpt, 25);
    } else {
        $excerpt = get_bloginfo('description');
    }
    ?>

    <meta property="og:title" content="<?php echo get_the_title(); ?>"/>
    <meta property="og:description" content="<?php echo $excerpt; ?>"/>
    <meta property="og:type" content="article"/>
    <meta property="og:url" content="<?php echo get_permalink(); ?>"/>
    <meta property="og:site_name" content="<?php echo get_bloginfo(); ?>"/>
    <meta property="og:image" content="<?php echo $img_src; ?>"/>

<?php
}
add_action('wp_head', 'fb_opengraph', 5);