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.
Note: 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.