Type declarations

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

Note:

When overriding a parent method, the child's method must match any return type declaration on the parent. If the parent doesn't define a return type, then the child method may do so.

Single types

Type Description Version
Class/interface name The value must be an instanceof the given class or interface.  
self The value must be an instanceof the same class as the one in which the type declaration is used. Can only be used in classes.  
parent The value must be an instanceof the parent of the class in which the type declaration is used. Can only be used in classes.  
array The value must be an array.  
callable The value must be a valid callable. Cannot be used as a class property type declaration.  
bool The value must be a boolean value.  
float The value must be a floating point number.  
int The value must be an integer.  
string The value must be a string.  
iterable The value must be either an array or an instanceof Traversable. PHP 7.1.0
object The value must be an object. PHP 7.2.0
mixed The value can be any value. PHP 8.0.0
Warning

Aliases for the above scalar types 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

mixed

mixed is equivalent to the union type object|resource|array|string|int|float|bool|null. Available as of PHP 8.0.0.

Examples

Example #1 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 #2 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 #3 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 #4 Returning an object

<?php
class C {}

function getC(): C {
    return new C;
}

var_dump(getC());
?>

The above example will output:

object(C)#1 (0) {
}

Nullable type

As of PHP 7.1.0, type declarations can be marked nullable by prefixing the type name with a question mark (?). This signifies that the value can be of the specified type or null.

Example #5 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 #6 Nullable return type declaration

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

Note:

It is possible to achieve nullable arguments by making null the default value. This is not recommended as this breaks during inheritance.

Example #7 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

Union types

A union type declaration accepts values of multiple different types, rather than a single one. Union types are specified using the syntax T1|T2|.... Union types are available as of PHP 8.0.0.

Nullable union types

The null type is supported as part of unions, such that T1|T2|null can be used to create a nullable union. The existing ?T notation is considered a shorthand for the common case of T|null.

Caution

null cannot be used as a standalone type.

false pseudo-type

The false literal type is supported as part of unions, and is included as for historical reasons many internal functions return false instead of null for failures. A classic example of such a function is strpos().

Caution

false cannot be used as a standalone type (including nullable standalone type). As such, all of false, false|null and ?false are not permitted.

Caution

The true literal type does not exist.

Duplicate and redundant types

To catch simple bugs in union 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 result in an error.
  • If bool is used, false 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.

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

use A as B;
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)
?>

Return only types

void

void is a return type indicating the function does not return a value. Therefore it cannot be part of a union type declaration. Available as of PHP 7.1.0.

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 {}
?>

never

never is a return type indicating the function does not return. This means that it either calls exit(), throws an exception, or is an infinite loop. Therefore it cannot be part of a union type declaration. Available as of PHP 8.1.0.

never is, in type theory parlance, the bottom type. Meaning it is the subtype of every other type and can replace any other return type during inheritance.

static

The value must be an instanceof the same class as the one the method is called in. Available as of PHP 8.0.0.

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 #8 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 #9 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 #10 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

Coercive typing with union types

When strict_types is not enabled, scalar type declarations are subject to limited implicit type coercions. If the exact type of the value is not part of the union, then the target type is chosen in the following order of preference:

  1. int
  2. float
  3. string
  4. bool
If the type both exists in the union, and the value can be coerced to the type under PHPs existing type checking semantics, then the type is chosen. Otherwise the next type is tried.

Caution

As an exception, if the value is a string and both int and float are part of the union, the preferred type is determined by the existing “numeric string” semantics. For example, for "42" int is chosen, while for "42.0" float is chosen.

Note:

Types that are not part of the above preference list are not eligible targets for implicit coercion. In particular no implicit coercions to the null and false types occur.

Example #11 Example of types being coerced into a type part of the union

<?php
// int|string
42    --> 42          // exact type
"42"  --> "42"        // exact type
new ObjectWithToString --> "Result of __toString()"
                      // object never compatible with int, fall back to string
42.0  --> 42          // float compatible with int
42.1  --> 42          // float compatible with int
1e100 --> "1.0E+100"  // float too large for int type, fall back to string
INF   --> "INF"       // float too large for int type, fall back to string
true  --> 1           // bool compatible with int
[]    --> TypeError   // array not compatible with int or string

// int|float|bool
"45"    --> 45        // int numeric string
"45.0"  --> 45.0      // float numeric string

"45X"   --> true      // not numeric string, fall back to bool
""      --> false     // not numeric string, fall back to bool
"X"     --> true      // not numeric string, fall back to bool
[]      --> TypeError // array not compatible with int, float or bool
?>

Misc

Example #12 Typed pass-by-reference Parameters

Declared types of reference parameters are checked on function entry, but not when the function returns, so after the function had returned, the argument's type may have changed.

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

Output of the above example in PHP 8:

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

Example #13 Catching TypeError

<?php
declare(strict_types=1);

function sum(int $a, int $b) {
    return $a + $b;
}

try {
    var_dump(sum(1, 2));
    var_dump(sum(1.5, 2.5));
} catch (TypeError $e) {
    echo 'Error: ', $e->getMessage();
}
?>

Output of the above example in PHP 8:

int(3)
Error: sum(): Argument #1 ($a) must be of type int, float given, called in - on line 10