Add image and total image count to the WordPress Gallery block

gallery-block-counters.php

<?php
/**
 * Add current image and total image count attrs to the gallery block.
 *
 * @param string $block_content The block content.
 * @param array  $block         The block type.
 * @param array  $instance      The block attributes.
 *
 * @return string The modified block content.
 */
function hd_utility_edit_gallery_markup( $block_content, $block, $instance ) {

	// create a new instance of the WP_HTML_Tag_Processor class.
	$tags = new WP_HTML_Tag_Processor( $block_content );

	// set figure counter.
	$count = 0;

	// loop through each image block.
	while ( $tags->next_tag( [ 'class_name' => 'wp-block-image' ] ) ) {

		// increment the count.
		$count++;

		// add an attribute for the current image count.
		$tags->set_attribute( 'data-image-current', $count );

		// set the total images count attribute.
		$tags->set_attribute( 'data-image-total', count( $block['innerBlocks'] ) );

	}

	// save the manipulated HTML back to the block content.
	$block_content = $tags->get_updated_html();

	// return the block content.
	return $block_content;

}

add_filter( 'render_block_core/gallery', 'hd_utility_edit_gallery_markup', 10, 3 );