Add custom port support for multisite.

sunrise.php

<?php
/**
 * Plugin Name:       Multisite custom port support
 * Plugin URI:        https://www.spacedmonkey.com
 * Description:       Enable custom port support for multisite.
 * Version:           1.0
 * Requires at least: 5.6
 * Requires PHP:      4.0
 * Author:            Jonathan Harris
 * Author URI:        https://www.spacedmonkey.com
 * License:           GPL v2 or later
 * License URI:       https://www.gnu.org/licenses/gpl-2.0.html
 */

/**
 * Hook into `pre_get_site_by_path` and look up site by port.
 *
 * @param null|false|WP_Site $site     Site value to return by path. Default null
 *                                     to continue retrieving the site.
 * @param null|false|WP_Site $site     Site value to return by path. Default null
 *                                     to continue retrieving the site.
 * @param string             $domain   The requested domain.
 * @param string             $path     The requested path, in full.
 * @param int|null           $segments The suggested number of paths to consult.
 *                                     Default null, meaning the entire path was to be consulted.
 * @param string[]           $paths    The paths to search for, based on $path and $segments.
 *
 * @return null|false|WP_Site $site     New site.
 */
function pre_get_site_by_path_with_port( $site, $domain, $path, $segments, $paths ) {
	$domain_pieces = parse_url( $domain );
	$domain        = $domain_pieces['host'];
	$domains       = array( $domain );
	if ( 'www.' === substr( $domain, 0, 4 ) ) {
		$domains[] = substr( $domain, 4 );
	}

	$args = array(
			'number'                 => 1,
			'update_site_meta_cache' => false,
	);

	if ( count( $domains ) > 1 ) {
		$args['domain__in']               = $domains;
		$args['orderby']['domain_length'] = 'DESC';
	} else {
		$args['domain'] = array_shift( $domains );
	}

	if ( count( $paths ) > 1 ) {
		$args['path__in']               = $paths;
		$args['orderby']['path_length'] = 'DESC';
	} else {
		$args['path'] = array_shift( $paths );
	}

	if ( $domain_pieces['port'] ) {
		$args['meta_query'] = array(
				array(
						'key'   => 'port',
						'value' => $domain_pieces['port']
				),
		);
	}

	$result = get_sites( $args );
	$site   = array_shift( $result );

	if ( $site ) {
		return $site;
	}

	return false;
}

add_filter( 'pre_get_site_by_path', 'pre_get_site_by_path_with_port', 10, 5 );


/**
 * Add form input to site edit form.
 *
 * @param int $site_id Site id.
 *
 * @return void
 */
function add_site_info_port( $site_id ) {
	$port = (string) get_site_meta( $site_id, 'port', true );
	?>
	<table class="form-table" role="presentation">
		<tr class="form-field">
			<th scope="row"><label for="url"><?php _e( 'Port' ); ?></label></th>
			<td><input name="port" type="number" id="port" value="<?php echo esc_attr( $port ); ?>"/></td>
		</tr>
	</table>
	<?php
}

add_action( 'network_site_info_form', 'add_site_info_port' );

/**
 * Hook into saving edit site to save port number.
 *
 * @param string $action
 * @param int $result
 *
 * @return void
 */
function edit_site_save_port( $action, $result ) {
	if ( $action === 'edit-site' && $result === 1 ) {
		$site_id = isset( $_POST['id'] ) ? wp_unslash( sanitize_text_field( $_POST['id'] ) ) : 0;
		$port    = isset( $_POST['port'] ) ? wp_unslash( sanitize_text_field( $_POST['port'] ) ) : '';
		update_site_meta( $site_id, 'port', $port );
	}
}
add_action( 'check_admin_referer', 'edit_site_save_port', 10, 2 );


/**
 * Hook into domain exists and lookup via port as well.
 *
 * @param int|null $old_result     The site ID if the site name exists, null otherwise.
 * @param string   $domain     Domain to be checked.
 * @param string   $path       Path to be checked.
 * @param int      $network_id Network ID. Relevant only on multi-network installations.
 *
 * @return mixed|null
 */
function domain_exists_port( $old_result, $domain, $path, $network_id ) {
	$port = isset( $_POST['port'] ) ? wp_unslash( sanitize_text_field( $_POST['port'] ) ) : 0;
	if ( ! $port ) {
		return $old_result;
	}

	$path   = trailingslashit( $path );
	$args   = array(
			'network_id'             => $network_id,
			'domain'                 => $domain,
			'path'                   => $path,
			'fields'                 => 'ids',
			'number'                 => 1,
			'update_site_meta_cache' => false,
			'meta_query'             => array(
					array(
							'key'   => 'port',
							'value' => $port
					),
			)
	);
	$result = get_sites( $args );
	$result = array_shift( $result );

	return $result;
}

add_filter( 'domain_exists', 'domain_exists_port', 10, 4 );

add_filter( 'redirect_network_admin_request', '__return_false' );