automatic-heading-ids.php

/**
 * Filters the_content to automatically add unique ID to headings.
 * 
 * Note: Change the regex to modify the related heading levels.
 * Defaults to levels 2 and 3.
 */
function who_automatic_heading_ids( $content ) {
	$pattern = "~<h(2|3)[^>]*>(.*?)</h(2|3)>~";
	$content = preg_replace_callback( $pattern, function ( $matches ) {
		$title = $matches[2];
		$slug  = sanitize_title_with_dashes( $title ) . '-' . uniqid();
		return "<h{$matches[1]} class='wp-block-heading' id='" . $slug . "'>" . $title . "</h{$matches[1]}>";
	}, $content );

	return $content;
}
add_filter( 'the_content', 'who_automatic_heading_ids' );