这是模板中的短代码：

function shortcode_frame_left( $atts, $content = null)
{

   return \'<span class="frame alignleft">\'. do_shortcode($content) . \'</span>\';
}
add_shortcode(\'frame_left\', \'shortcode_frame_left\');

以下是如何在帖子内容中使用：

[frame_left] <a href="YOUR-URL"><img src="YOUR-URL" /></a> [/frame_left]

我正在尝试在模板文件中使用此短代码来显示帖子缩略图。我的尝试：

if (has_post_thumbnail()) {

apply_filters( \'the_content\', "[frame_left]".the_post_thumbnail()."[/frame_left]"); 
}

以及

if (has_post_thumbnail()) {

$thumbnail = \'[frame_left]\'. the_post_thumbnail() . \'[/frame_left]\';
echo do_shortcode("$thumbnail");

}

我做错了什么？

if (has_post_thumbnail()) {
    // Echo the thumbnail
    the_post_thumbnail();
    // Apply the filter but do nothing with the result.
    apply_filters( \'the_content\', "[frame_left]"."[/frame_left]");
}

if (has_post_thumbnail()) {
    // Echo the thumbnail
    the_post_thumbnail();
    $thumbnail = \'[frame_left]\' . \'[/frame_left]\';
    // Echo the <span> with an empty content
    echo do_shortcode("$thumbnail");
}