## Type declarations

Type declarations can be added to function arguments, return values, as of PHP 7.4.0, class properties, and as of PHP 8.3.0, class constants. They ensure that the value is of the specified type at call time, otherwise a TypeError is thrown.

Every single type that PHP supports, with the exception of resource can be used within a user-land type declaration. This page contains a changelog of availability of the different types and documentation about usage of them in type declarations.

> Note:

> When a class implements an interface method or reimplements a method which has already been defined by a parent class, it has to be compatible with the aforementioned definition. A method is compatible if it follows the variance rules.

### Changelog

Version | Description
--- | ---
8.3.0 | Support for class, interface, trait, and enum constant typing has been added.
8.2.0 | Support for DNF types has been added.
8.2.0 | Support for the literal type true has been added.
8.2.0 | The types null and false can now be used standalone.
8.1.0 | Support for intersection types has been added.
8.1.0 | Returning by reference from a void function is now deprecated.
8.1.0 | Support for the return only type never has been added.
8.0.0 | Support for mixed has been added.
8.0.0 | Support for the return only type static has been added.
8.0.0 | Support for union types has been added.
7.4.0 | Support for class properties typing has been added.
7.2.0 | Support for object has been added.
7.1.0 | Support for iterable has been added.
7.1.0 | Support for void has been added.
7.1.0 | Support for nullable types has been added.

### Atomic Types Usage Notes

Atomic types have straight forward behaviour with some minor caveats which are described in this section.

#### Scalar types

Warning

Name aliases for scalar types (bool, int, float, string) are not supported. Instead, they are treated as class or interface names. For example, using `boolean` as a type declaration will require the value to be an `instanceof` the class or interface `boolean`, rather than of type bool:

`<?php
function test(boolean $param) {}
test(true);
?>`

Output of the above example in PHP 8:

    Warning: "boolean" will be interpreted as a class name. Did you mean "bool"? Write "\boolean" to suppress this warning in /in/9YrUX on line 2

    Fatal error: Uncaught TypeError: test(): Argument #1 ($param) must be of type boolean, bool given, called in - on line 3 and defined in -:2
    Stack trace:
    #0 -(3): test(true)
    #1 {main}
      thrown in - on line 2

#### void

> Note:

> Returning by reference from a void function is deprecated as of PHP 8.1.0, because such a function is contradictory. Previously, it already emitted the following `E_NOTICE` when called: Only variable references should be returned by reference.

> `<?php
> function &test(): void {}
> ?>`

#### Callable types

This type cannot be used as a class property type declaration.

> Note: It is not possible to specify the signature of the function.

#### Type declarations on pass-by-reference Parameters

If a pass-by-reference parameter has a type declaration, the type of the variable is only checked on function entry, at the beginning of the call, but not when the function returns. This means that a function can change the type of variable reference.

Example #1 Typed pass-by-reference Parameters

`<?php
function array_baz(array &$param)
{
$param = 1;
}
$var = [];
array_baz($var);
var_dump($var);
array_baz($var);
?>`

The above example will output something similar to:

    int(1)

    Fatal error: Uncaught TypeError: array_baz(): Argument #1 ($param) must be of type array, int given, called in - on line 9 and defined in -:2
    Stack trace:
    #0 -(9): array_baz(1)
    #1 {main}
      thrown in - on line 2

### Composite Types Usage Notes

Composite type declarations are subject to a couple of restrictions and will perform a redundancy check at compile time to prevent simple bugs.

Caution

Prior to PHP 8.2.0, and the introduction of DNF types, it was not possible to combine intersection types with union types.

#### Union types

Warning

It is not possible to combine the two singleton types false and true together in a union type. Use bool instead.

Caution

Prior to PHP 8.2.0, as false and null could not be used as standalone types, a union type comprised of only these types was not permitted. This comprises the following types: false, `false|null`, and `?false`.

##### Nullable type syntactic sugar

A single base type declaration can be marked nullable by prefixing the type with a question mark (`?`). Thus `?T` and `T|null` are identical.

> Note: This syntax is supported as of PHP 7.1.0, and predates generalized union types support.

> Note:

> It is also possible to achieve nullable arguments by making `null` the default value. This is not recommended as if the default value is changed in a child class a type compatibility violation will be raised as the null type will need to be added to the type declaration. This behavior is also deprecated since PHP 8.4.

> Example #2 Old way to make arguments nullable

> `<?php
> class C {}
> function f(C $c = null) {
> var_dump($c);
> }
> f(new C);
> f(null);
> ?>`

> The above example will output:

>     object(C)#1 (0) {
>     }
>     NULL

#### Duplicate and redundant types

To catch simple bugs in composite type declarations, redundant types that can be detected without performing class loading will result in a compile-time error. This includes:

- Each name-resolved type may only occur once. Types such as `int|string|INT` or `Countable&Traversable&COUNTABLE` result in an error.
- Using mixed or never results in an error.
- For union types:
  - If bool is used, false or true cannot be used additionally.
  - If object is used, class types cannot be used additionally.
  - If iterable is used, array and Traversable cannot be used additionally.
- For intersection types:
  - Using a type which is not a class-type results in an error.
  - Using either self, parent, or static results in an error.
- For DNF types:
  - If a more generic type is used, the more restrictive one is redundant.
  - Using two identical intersection types.

> Note: This does not guarantee that the type is “minimal”, because doing so would require loading all used class types.

For example, if `A` and `B` are class aliases, then `A|B` remains a legal union type, even though it could be reduced to either `A` or `B`. Similarly, if class `B extends A {}`, then `A|B` is also a legal union type, even though it could be reduced to just `A`.

`<?php
function foo(): int|INT {} // Disallowed
function foo(): bool|false {} // Disallowed
function foo(): int&Traversable {} // Disallowed
function foo(): self&Traversable {} // Disallowed
use A as B;
function foo(): A|B {} // Disallowed ("use" is part of name resolution)
function foo(): A&B {} // Disallowed ("use" is part of name resolution)
class_alias('X', 'Y');
function foo(): X|Y {} // Allowed (redundancy is only known at runtime)
function foo(): X&Y {} // Allowed (redundancy is only known at runtime)
?>`

### Examples

Example #3 Basic class type declaration

`<?php
class C {}
class D extends C {}
// This doesn't extend C.
class E {}
function f(C $c) {
echo get_class($c)."\n";
}
f(new C);
f(new D);
f(new E);
?>`

Output of the above example in PHP 8:

    C
    D

    Fatal error: Uncaught TypeError: f(): Argument #1 ($c) must be of type C, E given, called in /in/gLonb on line 14 and defined in /in/gLonb:8
    Stack trace:
    #0 -(14): f(Object(E))
    #1 {main}
      thrown in - on line 8

Example #4 Basic interface type declaration

`<?php
interface I { public function f(); }
class C implements I { public function f() {} }
// This doesn't implement I.
class E {}
function f(I $i) {
echo get_class($i)."\n";
}
f(new C);
f(new E);
?>`

Output of the above example in PHP 8:

    C

    Fatal error: Uncaught TypeError: f(): Argument #1 ($i) must be of type I, E given, called in - on line 13 and defined in -:8
    Stack trace:
    #0 -(13): f(Object(E))
    #1 {main}
      thrown in - on line 8

Example #5 Basic return type declaration

`<?php
function sum($a, $b): float {
return $a + $b;
}
// Note that a float will be returned.
var_dump(sum(1, 2));
?>`

The above example will output:

    float(3)

Example #6 Returning an object

`<?php
class C {}
function getC(): C {
return new C;
}
var_dump(getC());
?>`

The above example will output:

    object(C)#1 (0) {
    }

Example #7 Nullable argument type declaration

`<?php
class C {}
function f(?C $c) {
var_dump($c);
}
f(new C);
f(null);
?>`

The above example will output:

    object(C)#1 (0) {
    }
    NULL

Example #8 Nullable return type declaration

`<?php
function get_item(): ?string {
if (isset($_GET['item'])) {
return $_GET['item'];
} else {
return null;
}
}
?>`

Example #9 Class property type declaration

`<?php
class User {
public static string $foo = 'foo';
public int $id;
public string $username;
public function __construct(int $id, string $username) {
$this->id = $id;
$this->username = $username;
}
}
?>`

### Strict typing

By default, PHP will coerce values of the wrong type into the expected scalar type declaration if possible. For example, a function that is given an int for a parameter that expects a string will get a variable of type string.

It is possible to enable strict mode on a per-file basis. In strict mode, only a value corresponding exactly to the type declaration will be accepted, otherwise a TypeError will be thrown. The only exception to this rule is that an int value will pass a float type declaration.

Warning

Function calls from within internal functions will not be affected by the `strict_types` declaration.

To enable strict mode, the `declare` statement is used with the `strict_types` declaration:

> Note:

> Strict typing applies to function calls made from within the file with strict typing enabled, not to the functions declared within that file. If a file without strict typing enabled makes a call to a function that was defined in a file with strict typing, the caller's preference (coercive typing) will be respected, and the value will be coerced.

> Note:

> Strict typing is only defined for scalar type declarations.

Example #10 Strict typing for arguments values

`<?php
declare(strict_types=1);
function sum(int $a, int $b) {
return $a + $b;
}
var_dump(sum(1, 2));
var_dump(sum(1.5, 2.5));
?>`

Output of the above example in PHP 8:

    int(3)

    Fatal error: Uncaught TypeError: sum(): Argument #1 ($a) must be of type int, float given, called in - on line 9 and defined in -:4
    Stack trace:
    #0 -(9): sum(1.5, 2.5)
    #1 {main}
      thrown in - on line 4

Example #11 Coercive typing for argument values

`<?php
function sum(int $a, int $b) {
return $a + $b;
}
var_dump(sum(1, 2));
// These will be coerced to integers: note the output below!
var_dump(sum(1.5, 2.5));
?>`

The above example will output:

    int(3)
    int(3)

Example #12 Strict typing for return values

`<?php
declare(strict_types=1);
function sum($a, $b): int {
return $a + $b;
}
var_dump(sum(1, 2));
var_dump(sum(1, 2.5));
?>`

The above example will output:

    int(3)

    Fatal error: Uncaught TypeError: sum(): Return value must be of type int, float returned in -:5
    Stack trace:
    #0 -(9): sum(1, 2.5)
    #1 {main}
      thrown in - on line 5

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