WordPress filter to approve webmentions from previously-approved domains

functions.php

<?php

if ( !function_exists('indieweb_check_webmention') ) {

	/**
	 * Using the webmention_source_url, approve webmentions that have been received from previously-
	 * approved domains. For example, once you approve a webmention from http://example.com/post,
	 * future webmentions from http://example.com will be automatically approved.
	 * Recommend placing in your theme's functions.php
	 *
	 * Based on check_comment()
	 * @see https://core.trac.wordpress.org/browser/tags/4.9/src/wp-includes/comment.php#L113
	 */
	function indieweb_check_webmention($approved, $commentdata) {
		global $wpdb;

		if ( 1 == get_option('comment_whitelist')) {

			if ( !empty($commentdata['comment_meta']['webmention_source_url']) ) {
				$like_domain = sprintf('%s://%s%%', parse_url($commentdata['comment_meta']['webmention_source_url'], PHP_URL_SCHEME), parse_url($commentdata['comment_meta']['webmention_source_url'], PHP_URL_HOST));

				$ok_to_comment = $wpdb->get_var( $wpdb->prepare( "SELECT comment_approved FROM $wpdb->comments WHERE comment_author = %s AND comment_author_url LIKE %s AND comment_approved = '1' LIMIT 1", $commentdata['comment_author'], $like_domain ) );

				if ( 1 == $ok_to_comment ) {
					return 1;
				}

			}

		}

		return $approved;
	}

	add_filter('pre_comment_approved', 'indieweb_check_webmention', '99', 2);
}