Making a [gallery] in a loop with attachment IDs?

Question

I dont understand why this returns nothing but a blank screen.

But if you echo the $attachment->ID you get a string of ID numbers.

The goal is to produce a gallery with the wordpress [gallery] shortcode that includes all the featured images in all post.

$args = array( 'posts_per_page' => -1, 'offset'=> 1, 'post_type' => 'attachment' );

$posts = get_posts( $args );

foreach ( $posts as $attachment ) {
              do_shortcode('[gallery type="square" id="'.$attachment->ID.' ,"]');
    }

Also, which is it: ID or IDS (the codex shows both?!) and which is better to use ID or INCLUDE to pick which images to use?

NOTE: Tried this in default 20-13 and 20-14 themes, got the same blank results.

Answer 1

First get all of the feature thumbnail ids, and then populate the gallery. Code -

<?php
global $wpdb;
$featured_ids = $wpdb->get_col("SELECT meta_value FROM $wpdb->postmeta WHERE meta_key='_thumbnail_id'");
$featured_ids = array_map('intval', $featured_ids);
$featured_ids = array_unique($featured_ids);

echo do_shortcode('[gallery type="square" ids="'. implode(',', $featured_ids ).' ,"]');
?>