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Type System

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

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: - typebool - typeint - typefloat - typestring - typearray - typeobject - typeresource - typenever - typevoid - : , , and Relative class typesself``parent``static - Singleton types - false - true - Unit types - null
  • User-defined types (generally referred to as class-types) - Interfaces - Classes - Enumerations
  • typecallable

Scalar types

A value is considered scalar if it is of type ,
,  or .

int``float``string``bool

User-defined types

It is possible to define custom types with
,
 and
.
These are considered as user-defined types, or class-types.
For example, a class called  can be defined,
then objects of type  can be instantiated,
and a function can request a parameter of type .

interfacesclassesenumerationsElephant``Elephant``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 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 , , and
 will be written as .

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

|``T``U``V``T|U|V``T|(X&Y)

Type aliases

PHP supports two type aliases: and which corresponds to the

of and respectively. mixed``iterableunion typeobject|resource|array|string|float|int|bool|null``Traversable|array

Opmerking: > PHP does not support user-defined type aliases.

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