Source

File: wp-includes/ms-blogs.php

function get_id_from_blogname( $slug ) {
	$current_network = get_network();
	$slug            = trim( $slug, '/' );

	if ( is_subdomain_install() ) {
		$domain = $slug . '.' . preg_replace( '|^www\.|', '', $current_network->domain );
		$path   = $current_network->path;
	} else {
		$domain = $current_network->domain;
		$path   = $current_network->path . $slug . '/';
	}

	$site_ids = get_sites(
		array(
			'number'                 => 1,
			'fields'                 => 'ids',
			'domain'                 => $domain,
			'path'                   => $path,
			'update_site_meta_cache' => false,
		)
	);

	if ( empty( $site_ids ) ) {
		return null;
	}

	return array_shift( $site_ids );
}