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.
© 1997–2025 The PHP Documentation Group
Licensed under the Creative Commons Attribution License v3.0 or later.
https://www.php.net/manual/en/language.types.singleton.php