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.
Atomic types
Some atomic types are built-in types which are tightly integrated with the
language and cannot be reproduced with user-defined 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,
classes and
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
& 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 parentheses for it to be written in DNF:
T|(X&Y).
Type aliases
PHP supports two type aliases: mixed and
iterable which correspond to the
union type
of object|resource|array|string|float|int|bool|null
and Traversable|array respectively.
Introduction
Types