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.
Singleton types are those which allow only one value. PHP has support for two singleton types: false as of PHP 8.0.0 and true as of PHP 8.2.0.
Prior to PHP 8.2.0 the false type could only be used as part of a union type.
Nota: It is not possible to define custom singleton types. Consider using an enumerations instead.
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.