Object Interfaces

Class Abstraction
November 14, 2018
Overloading
November 14, 2018

Object interfaces allow you to create code which specifies which methods a class must implement, without having to define how these methods are implemented.

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.

Note that it is possible to declare a constructor in an interface, what can be useful in some contexts, e.g. for use by factories.

implements

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.

Constants

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

Examples

Example #1 Interface example

<?php

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

// Implement the interface
// This will work
class Template implements iTemplate
{
private $vars = array();

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 (iTemplate::getHtml)
class BadTemplate implements iTemplate
{
private $vars = array();

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

Example #2 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)
{
}
}
?>

Example #3 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()
{
}
}
?>

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