Types
PHP Manual

Singleton types

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.

Warning

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.