WP_CLI

Runner::is_multisite │ private │ WP-CLI 1.0

Whether or not this WordPress installation is multisite.

For use after wp-config.php has loaded, but before the rest of WordPress is loaded.

Method of the class: Runner{}

No Hooks.

Returns

null. Nothing (null).

Usage

// private - for code of main (parent) class only
$result = $this->is_multisite();

Runner::is_multisite() code WP-CLI 2.13.0-alpha

wp-cli/php/WP_CLI/Runner.php 
private function is_multisite() {
	if ( defined( 'MULTISITE' ) ) {
		return MULTISITE;
	}

	if ( defined( 'SUBDOMAIN_INSTALL' ) || defined( 'VHOST' ) || defined( 'SUNRISE' ) ) {
		return true;
	}

	return false;
}