====== Request for Comments: Invokable ======
  * Version: 1.0
  * Date: 2008-08-02
  * Author: Etienne Kneuss <colder@php.net>
  * Status: Under Discussion

===== Introduction =====

Closures were implemented in PHP_5_3. For them, an %%__invoke()%% magic method was implemented, but not only for closures, every classes can define their own %%__invoke()%% and then use $obj();

===== Proposal =====

Currently, there is no sane way to detect whether an object can be invoked or not. What this RFC proposes is an interface that covers %%__invoke()%%:

<code php>
interface Invokable {
   public function __invoke($args);
}

class Closure implements Invokable {
/* ... */
}

class MyClass implements Invokable { 
/* ... */
}
</code>
And make $obj(); possible only if the interface is implemented, and not only the method.
We have other examples of magic features that are covered by interfaces, like ArrayAccess or Iterator/Traversable.