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<?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');

//This will ensure that the proper doctype is added to our HTML. Without this code, most platforms would simply skip over our webpage, and the tags we are about to add would never get parsed. The next function is where we will actually add the proper metadata for Open Graph to work. The full function looks like this:

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( $post->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');
        }
?>
 
    <meta property="og:title" content="<?php echo the_title(); ?>"/>
    <meta property="og:description" content="<?php echo $excerpt; ?>"/>
    <meta property="og:type" content="article"/>
    <meta property="og:url" content="<?php echo the_permalink(); ?>"/>
    <meta property="og:site_name" content="<?php echo get_bloginfo(); ?>"/>
    <meta property="og:image" content="<?php echo $img_src; ?>"/>
 
<?php
    } else {
        return;
    }
}
add_action('wp_head', 'fb_opengraph', 5);