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Interfaces

Definition

In object-oriented programming, an interface or protocol type is a data type that acts as an abstraction of a class. It describes a set of method signatures, the implementations of which may be provided by multiple classes that are otherwise not necessarily related to each other. A class which provides the methods listed in an interface is said to implement the interface, or to adopt the protocol.

Interfaces are useful for encapsulation and reducing coupling. For example, in Java, the Comparable interface specifies the method compareTo. Thus, a sorting method only needs to take objects of types which implement Comparable to sort them, without knowing about the inner nature of the class (except that two of these objects can be compared via compareTo()).

– Wikipedia

Description

Object interfaces allow you to create code which specifies which methods and properties a class must implement, without having to define how these methods or properties are implemented. Interfaces share a namespace with classes, traits, and enumerations, so they may not use the same name.

Interfaces are defined in the same way as a class, but with the interface keyword replacing the class keyword and without any of the methods having their contents defined.

All methods declared in an interface must be public; this is the nature of an interface.

– PHP Reference

Example: Interface

<?php

interface SomeInterface
{
    public const SOME_CONSTANT = 'constant';

    public static function someStaticMethod(): string;
    public function someMethod(): string;
}

class SomeClass implements SomeInterface
{
    public static function someStaticMethod(): string
    {
        return 'static method';
    }

    public function someMethod(): string
    {
        return 'method';
    }
}

$someObject = new SomeClass();
print(
    SomeClass::SOME_CONSTANT . PHP_EOL
    . $someObject->someStaticMethod() . PHP_EOL
    . $someObject->someMethod() . PHP_EOL
);

Result (PHP 8.4):

constant
static method
method

Source code: Example

Example: Interface example

<?php

// Declare the interface 'Template'
interface Template
{
    public function setVariable($name, $var);
    public function getHtml($template);
}

// Implement the interface
// This will work
class WorkingTemplate implements Template
{
    private $vars = [];

    public function setVariable($name, $var)
    {
        $this->vars[$name] = $var;
    }

    public function getHtml($template)
    {
        foreach($this->vars as $name => $value) {
            $template = str_replace('{' . $name . '}', $value, $template);
        }

        return $template;
    }
}

// This will not work
// Fatal error: Class BadTemplate contains 1 abstract methods
// and must therefore be declared abstract (Template::getHtml)
class BadTemplate implements Template
{
    private $vars = [];

    public function setVariable($name, $var)
    {
        $this->vars[$name] = $var;
    }
}
?>

– PHP Reference

Interface and dependency injection

In practice, interfaces serve two complementary purposes:

• To allow developers to create objects of different classes that may be used interchangeably because they implement the same interface or interfaces. A common example is multiple database access services, multiple payment gateways, or different caching strategies. Different implementations may be swapped out without requiring any changes to the code that uses them.
• To allow a function or method to accept and operate on a parameter that conforms to an interface, while not caring what else the object may do or how it is implemented. These interfaces are often named like Iterable, Cacheable, Renderable, or so on to describe the significance of the behavior.

– PHP Reference

Example: Interface and dependency injection

<?php

interface Presentable
{
    public function getId(): int;
    public function getTitle(): string;
    public function getContent(): string;
}

class Note implements Presentable
{
    private static int $datumId = 0;

    public function __construct(
        private string $label,
        private string $text
    ) {
        ++self::$datumId;
    }

    public function getId(): int
    {
        return self::$datumId;
    }

    public function getTitle(): string
    {
        return $this->label;
    }

    public function getContent(): string
    {
        return $this->text;
    }
}

class Article implements Presentable
{
    public function __construct(
        private int $tag,
        private string $header,
        private string $body
    ) {
    }

    public function getId(): int
    {
        return $this->tag;
    }

    public function getTitle(): string
    {
        return $this->header;
    }

    public function getContent(): string
    {
        return $this->body;
    }
}

function display(Presentable $presentable)
{
    print(
        '#' . $presentable->getId()
        . ' "' . $presentable->getTitle() . '"' . PHP_EOL . PHP_EOL
        . $presentable->getContent() . PHP_EOL . PHP_EOL
    );
}

$someNote = new Note(
    'Python and prototyping',
    "Python is widely considered excellent for prototyping.\n"
    . "It is one of the most popular languages for rapid application development\n"
    . "because it allows developers to move from an idea to a working concept\n"
    . "much faster than lower-level languages like C++ or Java."
);

$someArticle = new Article(
    '1024',
    'C++ teaches more than any other programming language',
    "While modern languages like Python or Java automate many technical\n"
    . "details to improve developer productivity,\n"
    . "C++ leaves them in your hands, providing a deeper look at \"how computers think\"."
);

display($someNote);
display($someArticle);

Result (PHP 8.4):

#1 "Python and prototyping"

Python is widely considered excellent for prototyping.
It is one of the most popular languages for rapid application development
because it allows developers to move from an idea to a working concept
much faster than lower-level languages like C++ or Java.

#1024 "C++ teaches more than any other programming language"

While modern languages like Python or Java automate many technical
details to improve developer productivity,
C++ leaves them in your hands, providing a deeper look at "how computers think".

Source code: Example

An interface, together with type declarations, provides a good way to make sure that a particular object contains particular methods. See instanceof operator and type declarations.

– PHP Reference

Inplementing interface

implements operator

To implement an interface, the implements operator is used. All methods in the interface must be implemented within a class; failure to do so will result in a fatal error. Classes may implement more than one interface if desired by separating each interface with a comma.

Warning

A class that implements an interface may use a different name for its parameters than the interface. However, as of PHP 8.0 the language supports named arguments, which means callers may rely on the parameter name in the interface. For that reason, it is strongly recommended that developers use the same parameter names as the interface being implemented.

Note:

Interfaces can be extended like classes using the extends operator.

Note:

The class implementing the interface must declare all methods in the interface with a compatible signature. A class can implement multiple interfaces which declare a method with the same name. In this case, the implementation must follow the signature compatibility rules for all the interfaces. So covariance and contravariance can be applied.

Example: Implementing interface

<?php

interface Displayable
{
    public function getLabel(): string;
    public function getContent(): string;
}

class Datum implements Displayable
{
    public function __construct(
        private string $label,
        protected string $description
    ) {
    }

    public function getLabel(): string
    {
        return "Description: " . $this->description;
    }
}

$someDatum = new Datum(
    'Great operating system',
    'Linux is a great operating system for geeks, nerds and academics.'
);

print(
    $someDatum->getLabel() . PHP_EOL
    . $someDatum->getContent() . PHP_EOL
    . PHP_EOL
);

Result (PHP 8.4):

Description: De beneficiis lectionis
Core: In omnibus requiem quaesivi, et nusquam inveni nisi in angulo cum libro.

Source code: Example

Interface constant

It’s possible for interfaces to have constants. Interface constants work exactly like class constants. Prior to PHP 8.1.0, they cannot be overridden by a class/interface that inherits them.

– PHP Reference

Example: Interfaces with constants

<?php
interface A
{
    const B = 'Interface constant';
}

// Prints: Interface constant
echo A::B;


class B implements A
{
    const B = 'Class constant';
}

// Prints: Class constant
// Prior to PHP 8.1.0, this will however not work because it was not
// allowed to override constants.
echo B::B;
?>

– PHP Reference

Example: Interface constant

<?php

interface SomeInterface
{
    public const SOME_CONSTANT = 'constant';
}

class SomeClass implements SomeInterface
{
    public function someMethod(): void
    {
        print(self::SOME_CONSTANT . PHP_EOL);
    }
}

print(SomeInterface::SOME_CONSTANT . PHP_EOL);

$someObject = new SomeClass();
$someObject->someMethod();

Result (PHP 8.4):

constant
constant

Source code: Example

Interface method

Example: Interface method

<?php

interface SomeInterface
{
    public function someMethod(): string;
}

class SomeClass implements SomeInterface
{
    public function someMethod(): string
    {
        return 'method';
    }

    public function otherMethod(): void
    {
        print($this->someMethod() . PHP_EOL);
    }
}

$someObject = new SomeClass();
print($someObject->someMethod() . PHP_EOL);
$someObject->otherMethod();

Result (PHP 8.4):

method
method

Source code: Example

Interface property (hooks)

As of PHP 8.4.0, interfaces may also declare properties. If they do, the declaration must specify if the property is to be readable, writeable, or both. The interface declaration applies only to public read and write access.

A class may satisfy an interface property in multiple ways. It may define a public property. It may define a public virtual property that implements only the corresponding hook. Or a read property may be satisfied by a readonly property. However, an interface property that is settable may not be readonly.

Example: Interface properties example

<?php
interface I
{
    // An implementing class MUST have a publicly-readable property,
    // but whether or not it's publicly settable is unrestricted.
    public string $readable { get; }

    // An implementing class MUST have a publicly-writeable property,
    // but whether or not it's publicly readable is unrestricted.
    public string $writeable { set; }

    // An implementing class MUST have a property that is both publicly
    // readable and publicly writeable.
    public string $both { get; set; }
}

// This class implements all three properties as traditional, un-hooked
// properties. That's entirely valid.
class C1 implements I
{
    public string $readable;

    public string $writeable;

    public string $both;
}

// This class implements all three properties using just the hooks
// that are requested.  This is also entirely valid.
class C2 implements I
{
    private string $written = '';
    private string $all = '';

    // Uses only a get hook to create a virtual property.
    // This satisfies the "public get" requirement.
    // It is not writeable, but that is not required by the interface.
    public string $readable { get => strtoupper($this->writeable); }

    // The interface only requires the property be settable,
    // but also including get operations is entirely valid.
    // This example creates a virtual property, which is fine.
    public string $writeable {
        get => $this->written;
        set {
            $this->written = $value;
        }
    }

    // This property requires both read and write be possible,
    // so we need to either implement both, or allow it to have
    // the default behavior.
    public string $both {
        get => $this->all;
        set {
            $this->all = strtoupper($value);
        }
    }
}
?>

– PHP Reference

Example: Interface property (hooks)

<?php

interface SomeInterface
{
    public string $someProperty {
        set;
        get;
    }

    public string $someSetProperty {
        set;
    }

    public string $someGetProperty {
        get;
    }
}

class SomeClass implements SomeInterface
{
    public string $someProperty;

    public string $someSetProperty {
        set => $this->someGetProperty = $value . '>';
    }

    public string $someGetProperty = '' {
        get => '<' . $this->someGetProperty;
    }

    public function someMethod(string $value): void
    {
        $this->someSetProperty = $value;
    }

    public function otherMethod(): string
    {
        return $this->someProperty . ' ' . $this->someGetProperty;
    }
}

$someObject = new SomeClass();
$someObject->someProperty = 'some';
$someObject->someMethod('value');
print($someObject->otherMethod() . PHP_EOL);

Result (PHP 8.4):

some <value>

Source code: Example

Interface and magic methods

Interfaces may define magic methods to require implementing classes to implement those methods.

– PHP Reference

Interface and constructor

Although they are supported, including constructors in interfaces is strongly discouraged. Doing so significantly reduces the flexibility of the object implementing the interface. Additionally, constructors are not enforced by inheritance rules, which can cause inconsistent and unexpected behavior.

– PHP Reference

Multiple interface implementing

Example: Multiple interface implementing

<?php

interface SomeInterface
{
    public function someMethod(): string;
}

interface OtherInterface
{
    public function otherMethod(): string;
}

class SomeClass implements SomeInterface, OtherInterface
{
    public function someMethod(): string
    {
        return 'per speculum';
    }

    public function otherMethod(): string
    {
        return 'in aenigmate';
    }

    public function anotherMethod(): string
    {
        return
            'Videmus nunc ' . $this->someMethod()
            . ' et ' . $this->otherMethod() . '.';
    }
}

$someObject = new SomeClass();
print('Interfaces:' . PHP_EOL);
print_r(class_implements($someObject));
print('Some interface method result: ' . $someObject->someMethod() . PHP_EOL);
print('Other interface method result: ' . $someObject->otherMethod() . PHP_EOL);

print(PHP_EOL . $someObject->anotherMethod() . PHP_EOL);

Result (PHP 8.4):

Interfaces:
Array
(
    [SomeInterface] => SomeInterface
    [OtherInterface] => OtherInterface
)
Some interface method result: per speculum
Other interface method result: in aenigmate

Videmus nunc per speculum et in aenigmate.

Source code: Example

Interface extending interface

Example: Extendable interfaces

<?php
interface A
{
    public function foo();
}

interface B extends A
{
    public function baz(Baz $baz);
}

// This will work
class C implements B
{
    public function foo()
    {
    }

    public function baz(Baz $baz)
    {
    }
}

// This will not work and result in a fatal error
class D implements B
{
    public function foo()
    {
    }

    public function baz(Foo $foo)
    {
    }
}
?>

– PHP Reference

Example: Interface extending inteface

<?php

interface SomeInterface
{
    public function someMethod(): string;
}

interface OtherInterface extends SomeInterface
{
    public function otherMethod(): string;
}

class SomeClass implements OtherInterface
{
    public function someMethod(): string
    {
        return 'some method';
    }

    public function otherMethod(): string
    {
        return 'other method';
    }

    public function anotherMethod(): string
    {
        return
            'Videmus nunc ' . $this->someMethod()
            . ' et ' . $this->otherMethod() . '.';
    }
}

$someObject = new SomeClass();
print('Interfaces:' . PHP_EOL);
print_r(class_implements($someObject));
print('Extended interface method result: ' . $someObject->someMethod() . PHP_EOL);
print('Extending interface method result: ' . $someObject->otherMethod() . PHP_EOL);

print(PHP_EOL . $someObject->anotherMethod() . PHP_EOL);

Result (PHP 8.4):

Interfaces:
Array
(
    [OtherInterface] => OtherInterface
    [SomeInterface] => SomeInterface
)
Extended interface method result: some method
Extending interface method result: other method

Videmus nunc some method et other method.

Source code: Example

Example: Multiple interface inheritance

<?php
interface A
{
    public function foo();
}

interface B
{
    public function bar();
}

interface C extends A, B
{
    public function baz();
}

class D implements C
{
    public function foo()
    {
    }

    public function bar()
    {
    }

    public function baz()
    {
    }
}
?>

– PHP Reference

Extending class and implementing interface

Example: Extending and implementing simultaneously

<?php

class One
{
    /* ... */
}

interface Usable
{
    /* ... */
}

interface Updatable
{
    /* ... */
}

// The keyword order here is important. 'extends' must come first.
class Two extends One implements Usable, Updatable
{
    /* ... */
}
?>

– PHP Reference

Example: Extending class and implementing interface

<?php

interface Presentable
{
    public function getId(): int;
    public function getTitle(): string;
    public function getContent(): string;
}

abstract class Information
{
    protected static int $datumId = 0;

    public function __construct(
        protected string $label,
        protected string $text
    ) {
        self::$datumId = $this->processId(self::$datumId);
    }

    abstract protected static function processId(int $id): int;
}

class Article extends Information implements Presentable
{
    public function getId(): int
    {
        return self::$datumId;
    }

    public function getTitle(): string
    {
        return $this->label;
    }

    public function getContent(): string
    {
        return $this->text;
    }

    protected static function processId(int $id): int
    {
        return ++$id;
    }
}

function display(Presentable $presentable)
{
    print(
        '#' . $presentable->getId()
        . ' "' . $presentable->getTitle() . '"' . PHP_EOL . PHP_EOL
        . $presentable->getContent() . PHP_EOL . PHP_EOL
    );
}

$someArticle = new Article(
    'C++ teaches more than any other programming language',
    "While modern languages like Python or Java automate many technical\n"
    . "details to improve developer productivity,\n"
    . "C++ leaves them in your hands, providing a deeper look at \"how computers think\"."
);

display($someArticle);

Result (PHP 8.4):

#1 "C++ teaches more than any other programming language"

While modern languages like Python or Java automate many technical
details to improve developer productivity,
C++ leaves them in your hands, providing a deeper look at "how computers think".

Source code: Example

Interface implemented by abstract class

Example: Interfaces with abstract classes

<?php
interface A
{
    public function foo(string $s): string;

    public function bar(int $i): int;
}

// An abstract class may implement only a portion of an interface.
// Classes that extend the abstract class must implement the rest.
abstract class B implements A
{
    public function foo(string $s): string
    {
        return $s . PHP_EOL;
    }
}

class C extends B
{
    public function bar(int $i): int
    {
        return $i * 2;
    }
}
?>

– PHP Reference

Example: Interface implemented by abstract class

<?php

interface Presentable
{
    public function getId(): int;
    public function getTitle(): string;
    public function getContent(): string;
}

abstract class Information implements Presentable
{
    protected static int $datumId = 0;

    public function __construct(
        protected string $label,
        protected string $text
    ) {
        self::$datumId = $this->processId(self::$datumId);
    }

    abstract protected function processId(int $id): int;

    public function getId(): int
    {
        return self::$datumId;
    }

    public function getTitle(): string
    {
        return $this->label;
    }

    public function getContent(): string
    {
        return $this->text;
    }
}

class Article extends Information
{
    protected function processId(int $id): int
    {
        return ++$id;
    }
}

function display(Presentable $presentable)
{
    print(
        '#' . $presentable->getId()
        . ' "' . $presentable->getTitle() . '"' . PHP_EOL . PHP_EOL
        . $presentable->getContent() . PHP_EOL . PHP_EOL
    );
}

$someArticle = new Article(
    'C++ teaches more than any other programming language',
    "While modern languages like Python or Java automate many technical\n"
    . "details to improve developer productivity,\n"
    . "C++ leaves them in your hands, providing a deeper look at \"how computers think\"."
);

display($someArticle);

Result (PHP 8.4):

#1 "C++ teaches more than any other programming language"

While modern languages like Python or Java automate many technical
details to improve developer productivity,
C++ leaves them in your hands, providing a deeper look at "how computers think".

Source code: Example

Intefaces and variance compatibility

Example: Variance compatibility with multiple interfaces

<?php
class Foo {}
class Bar extends Foo {}

interface A {
    public function myfunc(Foo $arg): Foo;
}

interface B {
    public function myfunc(Bar $arg): Bar;
}

class MyClass implements A, B
{
    public function myfunc(Foo $arg): Bar
    {
        return new Bar();
    }
}
?>

– PHP Reference

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