Adds or updates user interface setting.
Both $name and $value can contain only ASCII letters, numbers, hyphens, and underscores.
This function has to be used before any output has started as it calls setcookie().
(string) (Required) The name of the setting.
(string) (Required) The value for the setting.
(bool|null) True if set successfully, false if not. Null if the current user can't be established.