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