我试图使用以下方法将函数的输出连接到帖子内容：

function bottom_content($content) {
    global $post;
    $content .= course_resources($post->ID);
    return $content;
}
add_filter(\'the_content\', \'bottom_content\');

function course_resources($id) {
    $course = get_post_meta($id); 
    $course[\'wpcw_associated_module\'][0];                       
    global $wpdb;

    // query for parent_id
    $course_id = $wpdb->get_row(\'SELECT \'.$wpdb->prefix.\'wpcw_courses.course_id FROM \'.$wpdb->prefix.\'wpcw_modules INNER JOIN \'.$wpdb->prefix.\'wpcw_courses ON \'.$wpdb->prefix.\'wpcw_modules.parent_course_id=\'.$wpdb->prefix.\'wpcw_courses.course_id WHERE \'.$wpdb->prefix.\'wpcw_modules.module_id =\'.$course["wpcw_associated_module"][0].\'\', ARRAY_A);
    // $course_id = $wpdb->get_row(\'SELECT * FROM \'.$wpdb->prefix.\'wpcw_modules INNER JOIN \'.$wpdb->prefix.\'wpcw_courses ON \'.$wpdb->prefix.\'wpcw_modules.parent_course_id=\'.$wpdb->prefix.\'wpcw_courses.course_id WHERE \'.$wpdb->prefix.\'wpcw_modules.module_id =\'.$course["wpcw_associated_module"][0].\'\', ARRAY_A);

    $upload_dir = wp_upload_dir();                      
    $resource_path = $upload_dir["basedir"].\'/resources/Sharepoint-Videos-Resources-\'. $course_id["course_id"].\'.zip\';

    if (file_exists($resource_path)) {
        $resource_link = $upload_dir["baseurl"]. \'/resources/Sharepoint-Videos-Resources-\'. $course_id["course_id"].\'.zip\';
        echo \'<div style="text-align:center;font-size:14px;background:#dedede;color:#777;border:1px solid #bbb;padding:20px 10px 20px;"><a href="\'. $resource_link .\'"><button style="font-size:16px;background:#4f8a3f;box-shadow:none;border:1px solid #3F803C;color:#f0f0f0;padding:10px 20px;"><i class="fa fa-download fa-1x"></i> Resource Files</button></a> </div>\';
    }
}

我错过了什么？