Source

File: wp-includes/ms-functions.php

function domain_exists( $domain, $path, $network_id = 1 ) {
	$path   = trailingslashit( $path );
	$args   = array(
		'network_id'             => $network_id,
		'domain'                 => $domain,
		'path'                   => $path,
		'fields'                 => 'ids',
		'number'                 => 1,
		'update_site_meta_cache' => false,
	);
	$result = get_sites( $args );
	$result = array_shift( $result );

	/**
	 * Filters whether a site name is taken.
	 *
	 * The name is the site's subdomain or the site's subdirectory
	 * path depending on the network settings.
	 *
	 * @since 3.5.0
	 *
	 * @param int|null $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 apply_filters( 'domain_exists', $result, $domain, $path, $network_id );
}