ページや投稿の抜粋からdescriptionを出力する

gistfile1.php

<?php

add_action('wp_head', 'add_description');
function add_description() {
    $meta = '<meta name="description" content="%s" />';
    if (is_home() || is_front_page()) {
        $site_description = get_bloginfo('description', 'display');
        if ($site_description) {
            printf($meta, $site_description);
        }
    } elseif (is_singular()) {
        $id = get_the_ID();
        $post = &get_post($id);
        $exc = null;
        if ($post->post_excerpt) {
            $exc = esc_attr($post->post_excerpt);
        } else {
            $exc = esc_attr(strip_tags($post->post_content));
            $exc = preg_replace("/[\r\n]/", " ", $exc);
            $exc = mb_strimwidth($exc, 0, 100, "...");
        }   
        if ($exc) {
            printf(
                $meta,
                $exc
            );  
        }   
    }   
}

?>