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 handled.

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.

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.

Note:
Prior to PHP 5.3.9, a class could not implement two interfaces that specified a method with the same name, since it would cause ambiguity. More recent versions of PHP allow this as long as the duplicate methods have the same signature.

Note:
Interfaces can be extended like classes using the extends operator.

Note:
The class implementing the interface must use the exact same method signatures as are defined in the interface. Not doing so will result in a fatal error.

Constants

It's possible for interfaces to have constants. Interface constants works 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()
    {
    }
}
?>

Example #4 Interfaces with constants


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

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


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

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