WordPress: Get Post Gallery Image IDs

get-post-gallery-image-ids.php

<?php
/**
 * Retrieves the image ids from a gallery.
 *
 * @param int|WP_Post $post Post ID or object.
 * @return array            An array of image ids.
 */
function get_gallery_image_ids( $post ) {
	$post = get_post( $post );
	if ( ! $post ) {
		return array();
	}

	if ( ! has_shortcode( $post->post_content, 'gallery' ) ) {
		return array();
	}

	$images_ids = array();
	if ( preg_match_all( '/' . get_shortcode_regex() . '/s', $post->post_content, $matches, PREG_SET_ORDER ) ) {
		foreach ( $matches as $shortcode ) {
			if ( 'gallery' === $shortcode[2] ) {
				$data = shortcode_parse_atts( $shortcode[3] );
				if ( ! empty( $data['ids'] ) ) {
					$images_ids = explode( ',', $data['ids'] );
				}
			}
		}
	}

	return $images_ids;
}

<?php
if ( get_post_gallery() ) :

    //Get the gallery object
    $gallery = get_post_gallery( get_the_ID(), false );

    //Form an array with the found ids
    $gallery_attachment_ids = explode( ',', $gallery['ids'] );

endif;

https://wordpress.stackexchange.com/questions/221733/how-to-get-id-of-images-used-in-gallery