PHP uses a nominal type system with a strong behavioral subtyping relation. The subtyping relation is checked at compile time whereas the verification of types is dynamically checked at run time.
PHP's type system supports various atomic types that can be composed together to create more complex types. Some of these types can be written astype declarations.
Some atomic types are built-in types which are tightly integrated with the language and cannot be reproduced with user defined types.
The list of base types is:
It is possible to define custom types withinterfaces,classes andenumerations. These are considered as user-defined types, or class-types. For example, a class calledElephant can be defined, then objects of typeElephant can be instantiated, and a function can request a parameter of typeElephant.
It is possible to combine multiple atomic types into composite types. PHP allows types to be combined in the following ways:
An intersection type accepts values which satisfies multiple class-type declarations, rather than a single one. Individual types which form the intersection type are joined by the& symbol. Therefore, an intersection type comprised of the typesT,U, andV will be written asT&U&V.
A union type accepts values of multiple different types, rather than a single one. Individual types which form the union type are joined by the| symbol. Therefore, a union type comprised of the typesT,U, andV will be written asT|U|V. If one of the types is an intersection type, it needs to be bracketed with parenthesis for it to written inDNF:T|(X&Y).
PHP supports two type aliases:mixed anditerable which corresponds to theunion type ofobject|resource|array|string|float|int|bool|null andTraversable|array respectively.
Note: PHP does not support user-defined type aliases.