Source

File: wp-includes/link-template.php

function get_site_url( $blog_id = null, $path = '', $scheme = null ) {
	if ( empty( $blog_id ) || ! is_multisite() ) {
		$url = get_option( 'siteurl' );
	} else {
		switch_to_blog( $blog_id );
		$url = get_option( 'siteurl' );
		restore_current_blog();
	}

	$url = set_url_scheme( $url, $scheme );

	if ( $path && is_string( $path ) ) {
		$url .= '/' . ltrim( $path, '/' );
	}

	/**
	 * Filters the site URL.
	 *
	 * @since 2.7.0
	 *
	 * @param string      $url     The complete site URL including scheme and path.
	 * @param string      $path    Path relative to the site URL. Blank string if no path is specified.
	 * @param string|null $scheme  Scheme to give the site URL context. Accepts 'http', 'https', 'login',
	 *                             'login_post', 'admin', 'relative' or null.
	 * @param int|null    $blog_id Site ID, or null for the current site.
	 */
	return apply_filters( 'site_url', $url, $path, $scheme, $blog_id );
}