php-doc-en/language/oop5/interfaces.xml

<?xml version="1.0" encoding="utf-8"?>
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 <sect1 xml:id="language.oop5.interfaces" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
  <title>Object Interfaces</title>
  <para>
   Object interfaces allow you to create code which specifies which methods a
   class must implement, without having to define how these methods are
   implemented.
  </para>
  <para>
   Interfaces are defined in the same way as a class, but with the <literal>interface</literal>
   keyword replacing the <literal>class</literal> keyword and without any of the methods having
   their contents defined.
  </para>
  <para>
   All methods declared in an interface must be public; this is the nature of an
   interface.
  </para>
  <para>
   Note that it is possible to declare a <link
   linkend="language.oop5.decon.constructor">constructor</link> in an interface,
   what can be useful in some contexts, e.g. for use by factories.
  </para>
  <sect2 xml:id="language.oop5.interfaces.implements">
   <title><literal>implements</literal></title>
   <para>
    To implement an interface, the <literal>implements</literal> 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.
   </para>
   <note>
    <para>
     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.
    </para>
   </note>
   <note>
    <para>
     Interfaces can be extended like classes using the <link linkend="language.oop5.inheritance">extends</link> 
     operator.
    </para>
   </note>
   <note>
    <para>
     The class implementing the interface must use a method signatures
     which is compatible with LSP (Liskov Substitution Principle).
     Not doing so will result in a fatal error.
     </para>
    </note>
  </sect2>
  <sect2 xml:id="language.oop5.interfaces.constants">
   <title><literal>Constants</literal></title>
   <para>
    It's possible for interfaces to have constants. Interface constants work exactly 
    like <link linkend="language.oop5.constants">class constants</link> except
    they cannot be overridden by a class/interface that inherits them.
   </para>
  </sect2>
  <sect2 xml:id="language.oop5.interfaces.examples">
   &reftitle.examples;
   <example xml:id="language.oop5.interfaces.examples.ex1">
    <title>Interface example</title>
     <programlisting role="php">
<![CDATA[
<?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;
    }
}
?>
]]>
    </programlisting>
   </example>
   <example xml:id="language.oop5.interfaces.examples.ex2">
    <title>Extendable Interfaces</title>
     <programlisting role="php">
<![CDATA[
<?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)
    {
    }
}
?>
]]>
     </programlisting>
   </example>
   <example xml:id="language.oop5.interfaces.examples.ex3">
    <title>Multiple interface inheritance</title>
     <programlisting role="php">
<![CDATA[
<?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()
    {
    }
}
?>
]]>
     </programlisting>
   </example>
   <example xml:id="language.oop5.interfaces.examples.ex4">
    <title>Interfaces with constants</title>
     <programlisting role="php">
<![CDATA[
<?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';
}
?>
]]>
     </programlisting>
   </example>
   <para>
     An interface, together with type-hinting, provides a good way to make sure
     that a particular object contains particular methods. See
     <link linkend="language.operators.type">instanceof</link> operator and
     <link linkend="language.oop5.typehinting">type hinting</link>.
   </para>
  </sect2>

 </sect1>
 
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