你想要的has_post_thumbnail.

add_filter( \'the_content\', insert_featured_image, 20 );
function insert_featured_image( $content ) {
  global $post;
  if (has_post_thumbnail($post->ID)) {
    $content = preg_replace( "/<\\/p>/", "</p>" . get_the_post_thumbnail($post->ID, \'post-single\'), $content, 1 );
  }
  return $content;
}

注：以上代码来自post referenced in the OP, 但在has_post_thumbnail 有条件的我还必须补充global $post; 避免“未定义变量”Noticess、 或者，你可以离开$post 引用输出为has_post_thumbnail 和get_the_post_thumbnail 将承担全球$post （这就是为什么原始代码可以工作的原因）。

add_filter( \'the_content\', insert_featured_image, 20 );
function insert_featured_image( $content ) {
  if (has_post_thumbnail()) {
    $content = preg_replace( "/<\\/p>/", "</p>" . get_the_post_thumbnail(null, \'post-single\'), $content, 1 );
  }
  return $content;
}