我写了一篇关于this here, 但以下是要点的总结：

• the_content 只能在“循环内”使用“循环内”只能通过调用setup_postdata() 和global $post.wp_reset_postdata()

代码

/**
 * Display the post content. Optinally allows post ID to be passed
 * @uses the_content()
 * @link http://stephenharris.info/get-post-content-by-id/
 * @link https://wordpress.stackexchange.com/questions/142957/use-the-content-outside-the-loop
 * @param int $id Optional. Post ID.
 * @param string $more_link_text Optional. Content for when there is more text.
 * @param bool $stripteaser Optional. Strip teaser content before the more text. Default is false.
 */
function sh_the_content_by_id( $post_id=0, $more_link_text = null, $stripteaser = false ){
    global $post;
    $post = get_post($post_id);
    setup_postdata( $post, $more_link_text, $stripteaser );
    the_content();
    wp_reset_postdata( $post );
}

您可以使用get_page() 或get_post() 从循环中获取内容

//For page
$page_id = 1;
$get_page_object = get_page( $page_id );
$page_object = $get_page_object->post_content;
echo $newpagecString = substr($page_object, 0, 100);
//For post
$post_id = 2;
$get_post_object = get_post( $post_id );
$post_object = $get_post_object->post_content;
echo $newpostcString = substr($post_object, 0, 100);