我正在开发一个WordPress插件，并试图在“相关帖子”部分的帖子下方显示/获取帖子缩略图。我尝试了很多本该有用但对我来说不合适的东西。。。这是代码(on my plugin.php file):

add_theme_support( \'post-thumbnails\', array( \'post\', \'page\' ) );

if ( has_post_thumbnail() ) {
    echo (\'there is a thumbnail\');
    echo get_the_post_thumbnail(get_the_ID(), "thumbnail"); // nothing
    echo get_the_post_thumbnail(); // nothing
    echo var_dump(get_the_post_thumbnail(get_the_ID(), "thumbnail")); //string \'\' (length=0)
    echo var_dump(get_the_post_thumbnail()); //string \'\' (length=0)
    the_post_thumbnail(\'thumbnail\'); // nothing
    the_post_thumbnail(); // nothing
}
else {
    echo (\'there is no thumbnail\');
}

首先，我认为我的帖子没有缩略图，但是my first echo returned "there is a thumbnail" 其余的都不会返回任何东西。。。所以我不明白。。。缺少什么？

谢谢

Edit:

以下是我的plugin.php 文件：

function Similar_Article ($content) {
    if (is_single() || is_page()) {
        the_post();
        $similar_to_add = \'\';
        $id = get_the_ID();
        $content_and_similar = $content . \'<br><br>\'; //spaces between post and related posts

        /* many methods called to find my related posts */

        if (has_post_thumbnail()){
            $content_and_similar .= get_the_post_thumbnail(); // it works
        }
        else {
            $imageData = wp_get_attachment_image_src(get_post_thumbnail_id ( $id ), \'thumbnail\'); 
            $content_and_similar .= \'<img src=" \'.$imageData[0].\' ">\';
        }

        $content_and_similar .= $similar_to_add;
        return $content_and_similar;
    }
    else {
        return $content;
    }
}

add_filter( \'the_content\', \'Similar_Article\' );

如果有缩略图，我的函数可以工作，但如果没有缩略图，则不显示任何内容。。。