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. 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. A class cannot implement two interfaces that share function names, since it would cause ambiguity. Interfaces can be extended like classes using the extends operator. 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. Its possible for interfaces to have constants. Interface constants works exactly like class constants. They cannot be overridden by a class/interface that inherits it. &reftitle.examples; 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; } } ?> ]]> ]]> ]]> ]]> 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.