setup_userdata

 * Used by wp_set_current_user() for back compat. Might be deprecated in the future.
 *
 * @since 2.0.4
 *
 * @global string  $user_login    The user username for logging in
 * @global WP_User $userdata      User data.
 * @global int     $user_level    The level of the user
 * @global int     $user_ID       The ID of the user
 * @global string  $user_email    The email address of the user
 * @global string  $user_url      The url in the user's profile
 * @global string  $user_identity The display name of the user
 *
 * @param int $for_user_id Optional. User ID to set up global data. Default 0.
 */
function setup_userdata( $for_user_id = 0 ) {
	global $user_login, $userdata, $user_level, $user_ID, $user_email, $user_url, $user_identity;

	if ( ! $for_user_id ) {
		$for_user_id = get_current_user_id();
	}
	$user = get_userdata( $for_user_id );

	if ( ! $user ) {
		$user_ID       = 0;
		$user_level    = 0;
		$userdata      = null;
		$user_login    = '';