implements
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.
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https://secure.php.net/manual/en/language.oop5.interfaces.php