# 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 atomic types that can be composed together to create more complex types. Some of these types can be written as [type declarations](#language.types.declarations).

## Atomic types

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:

- Built-in types

  - Scalar types:

    - `bool` type

    - `int` type

    - `float` type

    - `string` type

  - `array` type

  - `object` type

  - `resource` type

  - `never` type

  - `void` type

  - [Relative class types](#language.types.relative-class-types): `self`, `parent`, and `static`

  - [Singleton types](#language.types.singleton)

    - `false`

    - `true`

  - Unit types

    - `null`

- User-defined types (generally referred to as class-types)

  - [Interfaces](#language.oop5.interfaces)

  - [Classes](#language.oop5.basic.class)

  - [Enumerations](#language.types.enumerations)

- `callable` type

### Scalar types

A value is considered scalar if it is of type `int`, `float`, `string` or `bool`.

### User-defined types

It is possible to define custom types with [interfaces](#language.oop5.interfaces), [classes](#language.oop5.basic.class) and [enumerations](#language.types.enumerations). These are considered as user-defined types, or class-types. For example, a class called `Elephant` can be defined, then objects of type `Elephant` can be instantiated, and a function can request a parameter of type `Elephant`.

## Composite types

It is possible to combine multiple atomic 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 satisfy multiple class-type declarations, rather than a single one. Individual types which form the intersection type are joined by the `ENTITY` symbol. Therefore, an intersection type comprised of the types `T`, `U`, and `V` will be written as `TENTITYUENTITYV`.

### 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 parentheses for it to be written in DNF: `T|(XENTITYY)`.

## Type aliases

PHP supports two type aliases: `mixed` and `iterable` which correspond to the [union type](#language.types.type-system.composite.union) of `object|resource|array|string|float|int|bool|null` and `Traversable|array` respectively.

> [!NOTE]
> PHP does not support user-defined type aliases.