Get registered sites of a user in WordPress multisite.

registered-sites.php

<?php

/**
 * Get user's registered blogs/sites.
 *
 * @param int|bool $user_id User id or current user.
 *
 * @return array
 */
public static function get_registered_sites_of_user( $user_id = false ) {
	global $wpdb;

	$sites = array();

	// Make sure user id is ready.
	$user_id = empty( $user_id ) ? get_current_user_id() : $user_id;
	// Get user details.
	$user = get_userdata( $user_id );
	// We need user name.
	if ( empty( $user->user_login ) ) {
		return $sites;
	}

	// Get the signups of the user.
	$signups = $wpdb->get_results(
		$wpdb->prepare(
			'SELECT domain, path FROM ' . $wpdb->base_prefix . 'signups WHERE user_login = %s',
			$user->user_login
		)
	);

	// If sites are empty, return.
	if ( empty( $signups ) ) {
		return $sites;
	}

	// Loop through each site.
	foreach ( $signups as $signup ) {
		if ( isset( $signup->domain, $signup->path ) ) {
			$blog_id = get_blog_id_from_url( $signup->domain, $signup->path );
			if ( ! empty( $blog_id ) ) {
				$sites[] = $blog_id;
			}
		}
	}

	/**
	 * Filter for sites of a user.
	 *
	 * @param array $sites Sites of user.
	 */
	return apply_filters( 'get_registered_sites_of_user', $sites );
}