Auto-generate IDs for Heading blocks in WordPress

auto-heading-ids.php

<?php

add_filter('render_block_core/heading', function($content) {

	$processor = new \WP_HTML_Tag_Processor($content);

	// Find the block heading element and check if it has an ID.
	if (
		$processor->next_tag(['class_name' => 'wp-block-heading'])
		&& is_null($processor->get_attribute('id'))
	) {
		// Set a bookmark for the found heading element.
		$processor->set_bookmark('heading');

		// Look for the text token and use it to generate ID.
		while ($processor->next_token()) {
			if (
				'#text' === $processor->get_token_name()
				&& $text = $processor->get_modifiable_text()
			) {
				// Return to the heading bookmark.
				$processor->seek('heading');

				// Set ID for heading based on text.
				$processor->set_attribute(
					'id',
					sanitize_title_with_dashes($text)
				);

				// Break out of loop.
				break;
			}
		}
	}

	return $processor->get_updated_html();
});