对象接口

使用接口（interface），可以指定某个类必须实现哪些方法，但不需要定义这些方法的具体内容。

接口是通过 interface 关键字来定义的，就像定义一个标准的类一样，但其中定义所有的方法都是空的。

接口中定义的所有方法都必须是公有，这是接口的特性。

需要注意的是，在接口中定义一个构造方法是被允许的。在有些场景下这可能会很有用，例如用于工厂模式时。

实现（implements）

要实现一个接口，使用 implements 操作符。类中必须实现接口中定义的所有方法，否则会报一个致命错误。类可以实现多个接口，用逗号来分隔多个接口的名称。

Note:

在 PHP 5.3.9 之前，实现多个接口时，接口中的方法不能有重名，因为这可能会有歧义。在最近的 PHP 版本中，只要这些重名的方法签名相同，这种行为就是允许的。

Note:

接口也可以继承，通过使用 extends 操作符。

Note:

类要实现接口，必须使用和接口中所定义的方法完全一致的方式。否则会导致致命错误。

常量

接口中也可以定义常量。接口常量和类常量的使用完全相同，但是不能被子类或子接口所覆盖。

范例

Example #1 接口示例

<?php

// 声明一个'iTemplate'接口
interface iTemplate
{
    public function setVariable($name, $var);
    public function getHtml($template);
}


// 实现接口
// 下面的写法是正确的
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;
    }
}

// 下面的写法是错误的，会报错，因为没有实现 getHtml()：
// 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 可扩充的接口

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

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

// 正确写法
class c implements b
{
    public function foo()
    {
    }

    public function baz(Baz $baz)
    {
    }
}

// 错误写法会导致一个致命错误
class d implements b
{
    public function foo()
    {
    }

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

Example #3 继承多个接口

<?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 使用接口常量

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

// 输出接口常量
echo a::b;

// 错误写法，因为常量不能被覆盖。接口常量的概念和类常量是一样的。
class b implements a
{
    const b = 'Class constant';
}
?>

接口加上类型约束，提供了一种很好的方式来确保某个对象包含有某些方法。参见 instanceof 操作符和类型约束。

User Contributed Notes

naderi dot payam at gmail dot com 15-Oct-2015 07:57
<?php
    interface ClassThatCanBuild
    { }

    interface SpecificClass extends \ClassThatCanBuild
    { }

    Class MySpecificClass implements \SpecificClass
    { }

    interface BuilderInterface
    {
        function build(\ClassThatCanBuild $container);

        function weCanHaveMethods();
    }

    interface SpecificBuilderInterface extends \BuilderInterface
    {
        /**
         * (!!) this class want to allow just for build
         *      specific classes.
         *      i`m still on the rule with MySpecificClass
         *      because extend from ClassThatCanBuild
         *      but php don`t allow it at all
         *
         *      "" declaration of must be compatible with
         *         interface of BuilderInterface->build ""
         *
         * @param \MySpecificClass $container
         */
        function build(\MySpecificClass $container);
    }
?>
naderi dot payam at gmail dot com 19-Sep-2015 12:44
php inheritance interfaces on function/methods type declarations:

<?php
interface iInvokable {
        function __invoke($arg = null);
    }

    interface iResponder extends iInvokable {
        /** Bind next responder */
        function then(iInvokable $responder);
    }

    class Responder implements \iResponder {

        function __invoke($arg = null)
        {
            // TODO: Implement __invoke() method.
        }

        /** Bind next responder */
        function then(iInvokable $responder)
        {
            // TODO: Implement then() method.
        }
    }

    class OtherResponder implements \iResponder {

        function __invoke($arg = null)
        {
            // TODO: Implement __invoke() method.
        }

        /** Bind next responder */
        function then(iInvokable $responder)
        {
            // TODO: Implement then() method.
        }
    }

    class Invokable implements \iInvokable {

        function __invoke($arg = null)
        {
            // TODO: Implement __invoke() method.
        }
    }

    $responder = new Responder();
    $responder->then(new OtherResponder());
    $responder->then(new Invokable());
?>
aditycse at gmail dot com 21-Jul-2015 05:50
Solution for overriding interface constants
//we can override interface constants by using  abstract class
//if Test Class implements inter1 than we can not override inter1 interface constants
example:

<?php

interface inter1 {

    const interface1 = "I am from interface 1";

    function foo1();

    function bar1();
}

interface inter2 extends inter1 {

    function foo2();

    function bar2();
}

interface inter3 {

    function foo3();

    function bar3();
}

interface inter4 {

    function foo4();

    function bar4();
}

abstract class AbsClass implements inter2 {
   
}

class Test extends AbsClass implements inter3, inter4 {

    const interface1 = "I am from test class";

    public function foo1() {
       
    }

    public function foo2() {
       
    }

    public function foo3() {
       
    }

    public function foo4() {
       
    }

    public function bar1() {
       
    }

    public function bar2() {
       
    }

    public function bar3() {
       
    }

    public function bar4() {
       
    }

    public function display() {
        echo inter1::interface1;
        echo PHP_EOL;
        echo Test::interface1;
    }

}

$Obj = new Test();
$Obj->display(); //I am from interface 1 \n I am from test class
koalay at gmail dot com 27-Nov-2014 04:37
Just wrote some examples of duck-typing in PHP. Sharing here.

<?php

/**
 * An example of duck typing in PHP
 */

interface CanFly {
  public function fly();
}

interface CanSwim {
  public function swim();
}

class Bird {
  public function info() {
    echo "I am a {$this->name}\n";
    echo "I am an bird\n";
  }
}

/**
 * some implementations of birds
 */
class Dove extends Bird implements CanFly {
  var $name = "Dove";
  public function fly() {
    echo "I fly\n";
  }
}

class Penguin extends Bird implements CanSwim {
  var $name = "Penguin";
  public function swim() {
    echo "I swim\n";
  }
}

class Duck extends Bird implements CanFly, CanSwim {
  var $name = "Duck";
  public function fly() {
    echo "I fly\n";
  }
  public function swim() {
    echo "I swim\n";
  }
}

/**
 * a simple function to describe a bird
 */
function describe($bird) {
  if ($bird instanceof Bird) {
    $bird->info();
    if ($bird instanceof CanFly) {
      $bird->fly();
    }
    if ($bird instanceof CanSwim) {
      $bird->swim();
    }
  } else {
    die("This is not a bird. I cannot describe it.");
  }
}

// describe these birds please
describe(new Penguin);
echo "---\n";

describe(new Dove);
echo "---\n";

describe(new Duck);
marcoshoya at gmail dot com 31-Aug-2014 08:48
You can change some methods signature from interface if you pass parameters with default values. For instance:

<?php

interface Foo {
  public function foo($foo);
}

class Bar implements Foo {
  public function foo($foo, $bar = null) {
    return $foo . $bar;
  }
}

$bar = new Bar();
// output: worksfine
echo $bar->foo('works', 'fine');
williebegoode at att dot net 10-Aug-2014 10:42
In their book on Design Patterns, Erich Gamma and his associates (AKA: "The Gang of Four") use the term "interface" and "abstract class" interchangeably. In working with PHP and design patterns, the interface, while clearly a "contract" of what to include in an implementation is also a helpful guide for both re-use and making changes. As long as the implemented changes follow the interface (whether it is an interface or abstract class with abstract methods), large complex programs can be safely updated without having to re-code an entire program or module.

In PHP coding with object interfaces (as a keyword) and "interfaces" in the more general context of use that includes both object interfaces and abstract classes, the purpose of "loose binding" (loosely bound objects) for ease of change and re-use is a helpful way to think about both uses of the  term "interface." The focus shifts from "contractual" to "loose binding" for the purpose of cooperative development and re-use.
thomas tulinsky 25-Jun-2014 09:37
to dlovell2011 at yahoo dot com wrote:
It seems like many contributors are missing the point of using an INTERFACE. An INTERFACE is not specifically provided for abstraction.
...
An INTERFACE is provided so you can describe a set of functions and then hide the final implementation of those functions in an implementing class. This allows you to change the IMPLEMENTATION of those functions without changing how you use it.
--------------
I believe the unique thing that Interfaces provide is to support the same functions in different classes that are **not descended from a common ancestor**. 
Different implementations of the same functions does not need interfaces--it can be achieved through subclasses.
Vasiliy Makogon 06-Feb-2014 11:38
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. -- this documentation page.

But, if you use default values in arguments in methods, fatal error is not follow:

<?php
interface myInterface {
    public function __construct();
}

class concret implements myInterface {

    public function __construct($arg=null)
    {
        print_r(func_get_args());
    }
}

$obj = new concret(123);
?>

Array ( [0] => 123 )
tkostantinov 02-Dec-2013 10:58
Implementation must be strict, subtypes are not allowed.

"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. "

<?php

interface I
{
  function foo(stdClass $arg);
}

class Test extends stdClass
{
}

class Implementation implements I
{
  function foo(Test $arg)
    {
    }
}
?>
Result:

Fatal error: Declaration of InterfaceImplementation::foo() must be compatible with I::foo(stdClass $arg) in test.php on line XY
Vladimir Kornea 15-Jul-2013 10:55
By asking your colleague to implement your interface, you are asking him to guarantee that he has implemented all the methods you need. He can implement multiple interfaces, each being a contract, so that the guy cleaning the cruft down the road can see which methods belong to which area of concern; but since interfaces are primarily contracts rather than documentation, a class cannot implement two interfaces containing the same method since that would make the contract lose credibility through ambiguity. Thus a class can implement two interfaces with overlapping concerns only by factoring their common methods into a third interface that serves as a base for the first two, which resolves the contractual ambiguity.
Vladimir Kornea 26-Jun-2013 07:34
By asking your colleague to implement your interface, you are asking him to guarantee that he has implemented all the methods you need. He can implement multiple interfaces, each being a contract, so that the guy cleaning up the cruft down the road can see which methods belong to which area of concern; but since interfaces are primarily contracts rather than documentation, a class cannot implement two interfaces containing the same method since that would make the contract lose credibility through ambiguity. Thus a class can implement two interfaces with overlapping concerns only by factoring their common methods into a third interface that serves as a base for the first two, which resolves the contractual ambiguity.
dlovell2001 at yahoo dot com 03-Feb-2012 10:40
It seems like many contributors are missing the point of using an INTERFACE. An INTERFACE is not specifically provided for abstraction. That's what a CLASS is used for. Most examples in this article of interfaces could be achieved just as easily using just classes alone.

An INTERFACE is provided so you can describe a set of functions and then hide the final implementation of those functions in an implementing class. This allows you to change the IMPLEMENTATION of those functions without changing how you use it.

For example: I have a database. I want to write a class that accesses the data in my database. I define an interface like this:

interface Database {
function listOrders();
function addOrder();
function removeOrder();
...
}

Then let's say we start out using a MySQL database. So we write a class to access the MySQL database:

class MySqlDatabase implements Database {
function listOrders() {...
}
we write these methods as needed to get to the MySQL database tables. Then you can write your controller to use the interface as such:

$database = new MySqlDatabase();
foreach ($database->listOrders() as $order) {

Then let's say we decide to migrate to an Oracle database. We could write another class to get to the Oracle database as such:

class OracleDatabase implements Database {
public function listOrders() {...
}

Then - to switch our application to use the Oracle database instead of the MySQL database we only have to change ONE LINE of code:

$database = new OracleDatabase();

all other lines of code, such as:

foreach ($database->listOrders() as $order) {

will remain unchanged. The point is - the INTERFACE describes the methods that we need to access our database. It does NOT describe in any way HOW we achieve that. That's what the IMPLEMENTing class does. We can IMPLEMENT this interface as many times as we need in as many different ways as we need. We can then switch between implementations of the interface without impact to our code because the interface defines how we will use it regardless of how it actually works.
julien arobase fastre point info 12-Dec-2011 11:12
If you use namespaces and autoloading, do not forget to mention the use statement for each class used in yours arguments, before your class:

<?php
#file : fruit/squeezable.php
namespace fruit
use Bar\Foo;

interface squeezable {
  public function squeeze (Foo $foo);
}
?>

<?php
#file: orange

namespace fruit\citrus;

class orange {
  public function squeeze(Foo $foo);
}
#Will throw an exception Fatal error: Declaration of "fruit\citrus\orange::squeeze must be compatible with that of fruit\squeezable() in fruit/squeezable.php
?>

<?php
#file: orange

namespace fruit\citrus;
use Bar\Foo; #DO NOT FORGET THIS!

class orange {
  public function squeeze (Foo $foo);
}
#Will be correct
?>
thanhn2001 at gmail dot com 03-Mar-2011 01:37
PHP prevents interface a contant to be overridden by a class/interface that DIRECTLY inherits it.  However, further inheritance allows it.  That means that interface constants are not final as mentioned in a previous comment.  Is this a bug or a feature?

<?php

interface a
{
    const b = 'Interface constant';
}

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

class b implements a
{
}

// This works!!!
class c extends b
{
    const b = 'Class constant';
}

echo c::b;
?>
jballard at natoga dot com 06-Jan-2011 01:33
If it isn't already obvious, you can create an object of a class above the class declaration if it does NOT implement an interface. However, when a class DOES implement an interface, PHP will throw a "class not found" error unless the instantiation declaration is _below_ the class definition.

<?php
$bar = new foo(); // Valid

class foo
{
   
}
?>

<?php
$bar = new foo(); // Invalid - throws fatal error

interface foo2
{

}

class bar implements foo2
{
   
}

$bar = new foo(); // Valid, since it is below the class declaration

?>

Also, @Jeffrey -- Lol? What? Is that a joke? PHP interfaces have nothing to do with connecting to "peripheral devices" or "cameras", etc. Not even in the least sense. This is a very common miscommunication with the word "interface", as interfaces in programming are not at all like interfaces in electronics or drivers, etc.
Anonymous 13-Aug-2010 12:44
If you want to ensure implementation classes are correctly initialised (i.e. due to trickery one needs to do to work around lack of multiple inheritance), simply add  __construct() to your interface, so risk of init being forgotten is reduced.
drieick at hotmail dot com 23-Feb-2010 09:29
I was wondering if implementing interfaces will take into account inheritance. That is, can inherited methods be used to follow an interface's structure?

<?php

interface Auxiliary_Platform {
    public function Weapon();
    public function Health();
    public function Shields();
}

class T805 implements Auxiliary_Platform {
    public function Weapon() {
        var_dump(__CLASS__);
    }
    public function Health() {
        var_dump(__CLASS__ . "::" . __FUNCTION__);
    }
    public function Shields() {
        var_dump(__CLASS__ . "->" . __FUNCTION__);
    }
}

class T806 extends T805 implements Auxiliary_Platform {
    public function Weapon() {
        var_dump(__CLASS__);
    }
    public function Shields() {
        var_dump(__CLASS__ . "->" . __FUNCTION__);
    }
}

$T805 = new T805();
$T805->Weapon();
$T805->Health();
$T805->Shields();

echo "<hr />";

$T806 = new T806();
$T806->Weapon();
$T806->Health();
$T806->Shields();

/* Output:
string(4) "T805"
string(12) "T805::Health"
string(13) "T805->Shields"
<hr />string(4) "T806"
string(12) "T805::Health"
string(13) "T806->Shields"
*/

?>

Class T805 implements the interface Auxiliary_Platform. T806 does the same thing, but the method Health() is inherited from T805 (not the exact case, but you get the idea). PHP seems to be fine with this and everything still works fine. Do note that the rules for class inheritance doesn't change in this scenario.

If the code were to be the same, but instead T805 (or T806) DOES NOT implement Auxiliary_Platform, then it'll still work. Since T805 already follows the interface, everything that inherits T805 will also be valid. I would be careful about that. Personally, I don't consider this a bug.

This seems to work in PHP5.2.9-2, PHP5.3 and PHP5.3.1 (my current versions).

We could also do the opposite:

<?php

class T805 {
    public function Weapon() {
        var_dump(__CLASS__);
    }
}

class T806 extends T805 implements Auxiliary_Platform {
    public function Health() {
        var_dump(__CLASS__ . "::" . __FUNCTION__);
    }
    public function Shields() {
        var_dump(__CLASS__ . "->" . __FUNCTION__);
    }
}

$T805 = new T805();
$T805->Weapon();

echo "<hr />";

$T806 = new T806();
$T806->Weapon();
$T806->Health();
$T806->Shields();

/* Output:
string(4) "T805"
<hr />string(4) "T805"
string(12) "T806::Health"
string(13) "T806->Shields"
*/

?>

This works as well, but the output is different. I'd be careful with this.
uramihsayibok, gmail, com 10-Feb-2010 11:25
Interfaces can define static methods, but note that this won't make sense as you'll be using the class name and not polymorphism.

...Unless you have PHP 5.3 which supports late static binding:

<?php

interface IDoSomething {
    public static function doSomething();
}

class One implements IDoSomething {
    public static function doSomething() {
        echo "One is doing something\n";
    }
}

class Two extends One {
    public static function doSomething() {
        echo "Two is doing something\n";
    }
}

function example(IDoSomething $doer) {
    $doer::doSomething(); // "unexpected ::" in PHP 5.2
}

example(new One()); // One is doing something
example(new Two()); // Two is doing something

?>

If you have PHP 5.2 you can still declare static methods in interfaces. While you won't be able to call them via LSB, the "implements IDoSomething" can serve as a hint/reminder to other developers by saying "this class has a ::doSomething() method".
Besides, you'll be upgrading to 5.3 soon, right? Right?

(Heh. I just realized: "I do something". Unintentional, I swear!)
cretz 21-Sep-2008 03:49
FYI, interfaces can define constructors, destructors, and magic methods. This can be very helpful especially in the case of constructors when instantiating an implementing class via reflection in some sort of factory. Of course, it is not recommended to do such a thing since it goes against the nature of a true interface.
secure_admin 14-Sep-2008 02:28
In response to harryjry and mehea concerning your Weather Model. The problem is that you don't need all the things you think you need. In OOP, good class definitions get to the point rather quickly.

<?php
class Weather{
  public $time, $temperature, $humidity;

  public function __construct($tm, $t, $h){
    $this->time = $tm;
    $this->temperature = $t;
    $this->humidity = $h;
  }

  public function __toString(){
    return "Time: $this->time,
      Temperature: $this->temperature&deg;,
      Humidity: $this->humidity%";
  }
}

$forecasts = array(
  new Weather("1:00 pm", 65, 42),
  new Weather("2:00 pm", 66, 40),
  new Weather("3:00 pm", 68, 39)
  // add more weather reports as desired...
);
echo "Forecast for Chicago, IL:<br>";
foreach($forecasts as $forecast) echo ' - ' . $forecast '<br>';
?>
Forecast for Chicago, IL:
- Time: 1:00 pm, Temperature: 65°, Humidity: 42%
- Time: 2:00 pm, Temperature: 66°, Humidity: 40%
- Time: 3:00 pm, Temperature: 68°, Humidity: 39%

Note: MySQL can store data like this already, but if you included constants, more variables, and other functions in the Weather class, then maybe, just maybe it could be of use.
mehea 30-Jul-2008 11:55
While a subclass may implement an interface by extending an abstract class that implements the interface, I question whether it is good design to to do so.  Here's what I would suggest while taking the liberty of modifying the above weather/wet model:

<?php
interface water
{
    public function makeItWet();
}

 
/**
   * abstract class implements water but defines makeItWet
   * in the most general way to allow child class to
   * provide specificity
**/
abstract class weather implements water                   
{
   private $cloudy;
   public function makeItWet(){}
   abstract public function start();
   abstract public function getCloudy();
   abstract public function setCloudy();
}

class rain extends weather                            {
    private $cloudy;   
    public function start() {
        return "Here's some weather. ";
    }
   
    public function makeItWet() {
        return 'it is raining cats and dogs today.';
    }
    public function getCloudy() {
        return $this->cloudy;
    }
    public function setCloudy($bln=false) {
        $this->cloudy = $bln;
    }
}

$a = new rain();
echo $a->start();
$a->setCloudy(true);
if ($a->getCloudy()) {
    echo 'It is a cloudy day and ';
}
echo $a->makeItWet();

?>
harryjry at yahoo dot com 10-Jun-2008 02:29
The structure I am working with has a lot of inheritance going on, but not all methods are specified in one place. I needed a way to make sure an interface would be used, but that the method(s) defined in the interface are defined somewhere.

As such, I learned that the parent can define the interface's methods, and then the children can override that method at will without having to worry about the interface.

To expand on nrg1981's example, the following is possible:

<?php
interface water
{
    public function makeItWet();
}

class weather
{
    public function makeItWet()
    {
        return 'it may or may not be wet';
    }
   
    public function start()
    {
        return 'Here is some weather';
    }
}

class rain extends weather implements water
{
    public function makeItWet()
    {
        return 'It is wet';
    }
}

class thunder extends weather implements water
{

}

$a = new rain();
echo $a->start() . "\n";
echo $a->makeItWet() . "\n";

$a = new thunder();
echo $a->start() . "\n";
echo $a->makeItWet() . "\n";

?>
kaisershahid at gmail dot com 07-Apr-2008 05:41
php at wallbash dot com's comment of "It's important to note this because it is very unexpected behavior and renders many common Interface completly useless" doesn't make sense.

the idea of the interface is to force objects that aren't related to be reused in a common way. without them, to force that requirement, all objects that need those methods implemented would have to be descended from a base class that's known to have those methods. that's clearly not a smart idea if these objects aren't actually related.

one example (that i'm currently working on) is a background service that pulls information down from different content providers. i have a transport and i have an import. for both, what actually happens in the background is different from provider to provider, but since i'm implementing a transport & import interface, i only need to write code once, because i know exactly the what methods will be implemented to get the job done. then, i just have a config file that loads the class dynamically. i don't need something like

if ( $provider == "some company" )
{
   // use this set of code
}
elseif ( $provider == "another company" )
{
   // use this other set of code
}

instead, i can do:

foreach ( $providers as $provider => $info )
{
    $_transport = $info['transportObject'];
    $transport = new $_transport();
    $_import = $info['importObject'];
    $import = new $_import();
   
    $transport->setImporter( $import );
    $transport->retrieve();
}

it is expected behavior that when a class implements two interfaces that share one or more method names, an error is thrown, because interfaces don't relate to each other. if you want that sort of inferred behavior (i.e. A and B are different except for these shared methods), stick to [abstract] classes.

it sucks that interface methods might collide for some common types of tasks (get(), set(), etc.), so knowing that, design your interfaces with more unique method names.
michael dot martinek at gmail dot com 03-Jan-2008 08:18
In regards to what Hayley Watson is writing:

The "interface" is a method of enforcing that anyone who implements it must include all the functions declared in the interface. This is an abstraction method, since you cannot just declare a base class and do something like "public abstract function myTest();" and later on extend that class.

If you don't override the default value in a parameter list, it's assumed that the default value was received by time you have any control to read or relay the value on again. There should be no problem in having all or none of your parameters in an interface having a default value, as the value is "auto-filled" if not explicitly provided.

I just came across interfaces in PHP.. but I use them quite a bit in Java and Delphi. Currently building different DB wrappers, but all must enforce common access using a base class.. and also enforce that all of specific routines are implemented.
nrg1981 {AT} hotmail {DOT} com 05-Oct-2007 06:48
In case you would want to, a child class can implement an interface:

<?php

interface water
{
    public function makeItWet();
}

class weather
{
    public function start()
    {
        return 'Here is some weather';
    }
}

class rain extends weather implements water
{
    public function makeItWet()
    {
        return 'It is wet';
    }
}

$a = new rain();
echo $a->start();
echo $a->makeItWet();

?>
Hayley Watson 21-Sep-2007 03:42
If it's not already obvious, it's worth noticing that the parameters in the interface's method declaration do not have to have the same names as those in any of its implementations.

More significantly, default argument values may be supplied for interface method parameters, and they have to be if you want to use default argument values in the implemented classes:

<?php
interface isStuffable
{
    public function getStuffed($ratio=0.5);
}

class Turkey implements isStuffable
{
    public function getStuffed($stuffing=1)
    {
        // ....
    }
}
?>

Note that not only do the parameters have different names ($ratio and $stuffing), but their default values are free to be different as well. There doesn't seem to be any purpose to the interface's default argument value except as a dummy placeholder to show that there is a default (a class implementing isStuffable will not be able to implement methods with the signatures getStuffed(), getStuffed($a), or getStuffed($a,$b)).
Hayley Watson 20-Sep-2007 10:34
On an incidental note, it is not necessary for the implementation of an interface method to use the same variable names for its parameters that were used in the interface declaration.

More significantly, your interface method declarations can include default argument values. If you do, you must specify their implementations with default arguments, too. Just like the parameter names, the default argument values do not need to be the same. In fact, there doesn't seem to be any functionality to the one in the interface declaration at all beyond the fact that it is there.

<?php
interface isStuffed {
    public function getStuff($something=17);
}

class oof implements isStuffed {
    public function getStuff($a=42) {
        return $a;
    }
}

$oof = new oof;

echo $oof->getStuff();
?>

Implementations that try to declare the method as getStuff(), getStuff($a), or getStuff($a,$b) will all trigger a fatal error.
zedd at fadingtwilight dot net 04-Jul-2007 09:42
Regarding my previous note (04-Jul-2007 9:01):

I noticed a minor but critical mistake in my explanation. After the link to the PHP manual page on class abstraction, I stated:

"So by definition, you may only overload non-abstract methods."

This is incorrect. This should read:

"So by definition, you may only override non-abstract methods."

Sorry for any confusion.
Maikel 12-Jan-2007 03:07
if you want to implement an interface and in addition to use inheritance, first it uses "extends" and then "implements" example:

<?php
class MyChildClass extends MyParentClass implements MyInterface
{
   // definition
}
?>
Chris AT w3style DOT co.uk 07-Dec-2006 05:25
Note that you can extend interfaces with other interfaces since under-the-hood they are just abstract classes:

<?php

interface Foo {
    public function doFoo();
}

interface Bar extends Foo {
    public function doBar();
}

class Zip implements Bar {
    public function doFoo() {
        echo "Foo";
    }
    public function doBar() {
        echo "Bar";
    }
}

$zip = new Zip();
$zip->doFoo();
$zip->doBar();

?>

This is quite useful when you're using interfaces for identity more than the rigidity it places upon an API.  You can get the same result by implementing multiple interfaces.

An example of where I've used this in the past is with EventListener objects ala Java's Swing UI.  Some listeners are effectively the same thing but happen at different times therefore we can keep the same API but change the naming for clarity.
marasek AT telton POINT de 06-Sep-2006 06:01
What is not mentioned in the manual is that you can use "self" to force object hinting on a method of the implementing class:

Consider the following interface:
<?php
interface Comparable
{function compare(self $compare);}
?>

Which is then implemented:

<?php

class String implements Comparable
{
    private $string;
    function __construct($string)
    {$this->string = $string;}
    function compare(self $compare)
    {return $this->string == $compare->string;}
}

class Integer implements Comparable
{
    private $integer;
    function __construct($int)
    {$this->integer = $int;}
    function compare(self $compare)
    {return $this->integer == $compare->integer;}
}

?>

Comparing Integer with String will result in a fatal error, as it is not an instance of the same class:

<?php
$first_int = new Integer(3);
$second_int = new Integer(3);
$first_string = new String("foo");
$second_string = new String("bar");

var_dump($first_int->compare($second_int)); // bool(true)
var_dump($first_string->compare($second_string)); // bool(false)
var_dump($first_string->compare($second_int)); // Fatal Error
?>
darealremco at msn dot com 02-Oct-2005 12:53
To two notes below: There is one situation where classes and interfaces can be used interchangeably. In function definitions you can define parameter types to be classes or interfaces. If this was not so then there would not be much use for interfaces at all.
warhog at warhog dot net 11-Aug-2005 08:35
on the post below:

An interface is in fact the same like an abstract class containing abstract methods, that's why interfaces share the same namespace as classes and why therefore "real" classes cannot have the same name as interfaces.
marcus at synchromedia dot co dot uk 28-Jul-2005 04:11
Classes and interface names share a common name space, so you can't have a class and an interface with the same name, even though the two can never be used ambiguously (i.e. there are no circumstances in which a class and an interface can be used interchangeably). e.g. this will not work:

interface foo {
public function bling();
}

class foo implements foo {
public function bling() {
}
}

You will get a 'Cannot redeclare class' error, even though it's only been declared as a class once.
tobias_demuth at web dot de 04-May-2005 02:21
The statement, that you have to implement _all_ methods of an interface has not to be taken that seriously, at least if you declare an abstract class and want to force the inheriting subclasses to implement the interface.
Just leave out all methods that should be implemented by the subclasses. But never write something like this:

<?php

interface Foo {

      function bar();

}

abstract class FooBar implements Foo {

       abstract function bar(); // just for making clear, that this
                                 // method has to be implemented

}

?>

This will end up with the following error-message:

Fatal error: Can't inherit abstract function Foo::bar() (previously declared abstract in FooBar) in path/to/file on line anylinenumber
erik dot zoltan at msn dot com 25-Feb-2005 10:43
When should you use interfaces?  What are they good for?
Here are two examples. 

1. Interfaces are an excellent way to implement reusability. 
You can create a general interface for a number of situations
(such as a save to/load from disk interface.)  You can then
implement the interface in a variety of different ways (e.g. for
formats such as tab delimited ASCII, XML and a database.) 
You can write code that asks the object to "save itself to
disk" without having to worry what that means for the object
in question.  One object might save itself to the database,
another to an XML and you can change this behavior over
time without having to rewrite the calling code. 

This allows you to write reusable calling code that can work
for any number of different objects -- you don't need to know
what kind of object it is, as long as it obeys the common
interface. 

2. Interfaces can also promote gradual evolution.  On a
recent project I had some very complicated work to do and I
didn't know how to implement it.  I could think of a "basic"
implementation but I knew I would have to change it later. 
So I created interfaces in each of these cases, and created
at least one "basic" implementation of the interface that
was "good enough for now" even though I knew it would have
to change later. 

When I came back to make the changes, I was able to create
some new implementations of these interfaces that added the
extra features I needed.  Some of my classes still used
the "basic" implementations, but others needed the
specialized ones.  I was able to add the new features to the
objects themselves without rewriting the calling code in most
cases.  It was easy to evolve my code in this way because
the changes were mostly isolated -- they didn't spread all
over the place like you might expect.
mat.wilmots (at) wanadoo (dot) fr 20-Jan-2005 06:22
interfaces support multiple inheritance

<?php

interface SQL_Result extends SeekableIterator, Countable
{
    // new stuff
}

abstract class SQL_Result_Common
{
    // just because that's what one would do in reality, generic implementation
}

class SQL_Result_mysql extends SQL_Result_Common implements SQL_Result
{
   // actual implementation
}

?>

This code raises a fatal error because SQL_Result_mysql doesn't implement the abstract methods of SeekableIterator (6) + Countable (1)
russ dot collier at gmail dot com 28-Nov-2004 10:24
You can also specify class constants in interfaces as well (similar to specifying 'public static final' fields in Java interfaces):

<?php

interface FooBar
{

    const SOME_CONSTANT = 'I am an interface constant';

    public function doStuff();

}

?>

Then you can access the constant by referring to the interface name, or an implementing class, (again similar to Java) e.g.:

<?php

class Baz implements FooBar
{

    //....

}

print Baz::SOME_CONSTANT;
print FooBar::SOME_CONSTANT;

?>

Both of the last print statements will output the same thing: the value of FooBar::SOME_CONSTANT