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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 using the interface keyword, in the same way as a standard class, but 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 
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-hinting, provides a good way to make sure that a particular object contains particular methods. See instanceof operator and type hinting.

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