来源：http://lornajane.net/posts/2012/phps-magic-__invoke-method-and-the-callable-typehint

php 中的这个对象 ，有时间研究一下：

PHP中会有一些魔术方法：PHP 将所有以 __（两个下划线）开头的类方法保留为魔术方法。所以在定义类方法时，除了上述魔术方法，建议不要以 __ 为前缀。

魔术方法有：__construct()， __destruct()， __call()， __callStatic()， __get()， __set()， __isset()， __unset()， __sleep()，__wakeup()， __toString()， __invoke()， __set_state()， __clone() 和 __debugInfo() 等方法在 PHP 中被称为"魔术方法"（Magic methods）。在命名自己的类方法时不能使用这些方法名，除非是想使用其魔术功能。

Typehint

PHP has a variety of magic methods; methods named with two underscores at the start, which get called automatically when a particular event happens.（所以我标题中也叫事件方法，指特定的时间触发时，会自动调用，自动执行的方法。） In PHP 5.3, a new magic method was added: __invoke().

__invoke()

The __invoke() method gets called when the object is called as a function. When you declare it, you say which arguments it should expect. Here's a trivially simple example:

class Butterfly {

  public function __invoke() {

    echo "flutter";

  }

}

We can instantiate a Butterfly object, and then just use it like a function:

$bob = new Butterfly();

$bob(); // flutter

If you try to do the same thing on an object without an __invoke() method, you'll see this error:

PHP Fatal error:  Function name must be a string in filename.php on line X

We can check if the object knows how to be called by using the is_callable() function.

Callable Typehint

In PHP 5.4 (the newest version, and it has lots of shiny features), we have the Callable typehint. This allows us to check whether a thing is callable, either because it's a closure, an invokable object, or some other valid callback. Another trivial example to continue the butterflies and kittens theme:

也就是说， php 中 callable类型一共有3种： 

function sparkles(Callable $func) {

  $func();

  return "fairy dust";

}

class Butterfly {

  public function __invoke() {

    echo "flutter";

  }

}

$bob = new Butterfly();

echo sparkles($bob); // flutterfairy dust

So there it is, one invokable object being passed into a function and successfully passing a Callable typehint. I realise I also promised kittens, so here's a cute silver tabby I met the other day: