May be null could be considered to be a singleton type two. In my possibly wrong understanding, PHP has three singleton types, not just two.
单例类型（singleton type）是只允许单个值的类型。PHP 支持两种单例类型：false（自 PHP 8.0.0 起支持），true（自 PHP 8.2.0 起支持）。
注意: 无法定义自定义单例类型。建议改用枚举。
May be null could be considered to be a singleton type two. In my possibly wrong understanding, PHP has three singleton types, not just two.