Object Interfaces

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:

It is possible to declare magic methods such as the constructor in an interface, which 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.

Warning

A class can implement two interfaces which define a method with the same name, only if the method declaration in both interfaces is identical.

Note:

Interfaces can be extended like classes using the extends operator.

Note:

The class implementing the interface must declare a method which has a compatible signature.

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 
extends a
{
    public function 
baz(Baz $baz);
}

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

    public function 
baz(Baz $baz)
    {
    }
}

// This will not work and result in a fatal error
class 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 
extends ab
{
    public function 
baz();
}

class 
implements c
{
    public function 
foo()
    {
    }

    public function 
bar()
    {
    }

    public function 
baz()
    {
    }
}
?>

Example #4 Interfaces with constants

<?php
interface a
{
    const 
'Interface constant';
}

// Prints: Interface constant
echo a::b;


// This will however not work because it's not allowed to 
// override constants.
class implements a
{
    const 
'Class constant';
}
?>

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.