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Type System

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 base types that can be composed together to create more complex types. Some of these types can be written as type declarations.

Base types

Some base 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:

Composite types

It is possible to combine simple types into composite types. PHP allows types to be combined in the following ways:

  • Intersection of class-types (interfaces and class names).
  • Union of types.

Intersection types

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 types T, U, and V will be written as T&U&V.

Union types

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 types T, U, and V will be written as T|U|V. If one of the types is an intersection type, it needs to be bracketed with parenthesis for it to written in DNF: T|(X&Y).

Type aliases

PHP supports a two type aliases: mixed and iterable which corresponds to the union type of object|resource|array|string|float|int|bool|null and Traversable|array respectively.

Note: PHP does not support user-defined type aliases.

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