functions.php

// opengraph metatags
function danixland_opengraph() {
    global $post;
    $output = '';
    $image = get_template_directory_uri() . '/img/standard-logo.png';

    if ( is_home() || is_front_page() ) {
        $output  = '<meta property="og:url" content="' . get_bloginfo('url') . '" />' . "\n";
        $output .= '<meta property="og:type" content="website" />' . "\n";
        $output .= '<meta property="og:title" content="' . wp_title('&bull;', false, 'right') . '" />' . "\n";
        $output .= '<meta property="og:description" content="' . get_bloginfo( 'description' ) . '" />' . "\n";
        $output .= '<meta property="og:image" content="' . $image . '" />' . "\n";
    } elseif ( is_singular() ) {
        $output  = '<meta property="og:url" content="' . get_the_permalink() . '" />' . "\n";
        $output .= '<meta property="og:type" content="article" />' . "\n";
        $output .= '<meta property="og:title" content="' . get_the_title() . '" />' . "\n";
        $output .= '<meta property="og:description" content="' . get_the_excerpt() . '" />' . "\n";

        if ( is_single() && has_post_thumbnail($post->ID) ) {
            $image = get_the_post_thumbnail_url($post->ID, "medium");
        }
        $output .= '<meta property="og:image" content="' . $image . '" />' . "\n";
    }
    echo $output;
}
add_action( 'wp_head', 'danixland_opengraph' );