Concatenate a Function to the Post Content

Question

I am trying to use the following to concatenate a function's output to the post content:

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>';
    }
}

The desired outcome is for the <div> containing the button to be placed at the bottom of the post content, but no matter what priority I assign to the add_filter, the output is always at the top of the content.

What am I missing?

Answer 1

Your function needs to return the result. echo just outputs it into a page source and no result is produced to be concatenated.