Кофе против PHP:

###
 # @lang CoffeeScript
###
class Watcher
  @id         = 0
  @events     = []
  @observable = []

  @watch: (val = null) ->
    value = @observable[@id++] || undefined
    render= @events

    class Observable
      constructor: (val) ->
        value or=
          value:  undefined
          before: []
          after:  []

        return if val?
          fn value.value, val for fn in value.before
          value.value = val
          fn value.value, val for fn in value.after
          fn value.value, val for fn in render
        else
          value.value

      @before: (cb) ->
        value.before.push cb
        @

      @after: (cb) ->
        value.after.push cb
        @

      @subscribe: (cb) -> @after cb


    if val? then Observable val
    return Observable

  @subscribe: (cb) ->
    @events.push cb

window.watch = ((v) -> Watcher.watch(v))


# Вызов
a = watch 42
a.subscribe ->
  alert 'Оно изменилось!'
a 23




/**
 * @lang PHP
**/
class Watcher
{
    const EVT_BEFORE = 'before';
    const EVT_AFTER  = 'after';

    protected $value;

    protected $events = [
        self::EVT_BEFORE => [],
        self::EVT_AFTER  => []
    ];

    public function __construct($val)
    {
        $this->value = $val;
    }

    public function __invoke($val = null)
    {
        if ($val !== null) {
            foreach($this->events[self::EVT_BEFORE] as $e) { $e($this->value); }
            $this->value = $val;
            foreach($this->events[self::EVT_AFTER] as $e)  { $e($this->value); }
        }
        return $this->value;
    }

    public function before(callable $before)
    {
        $this->events[self::EVT_BEFORE][] = $before;
    }

    public function after(callable $after)
    {
        $this->events[self::EVT_AFTER][] = $after;
    }

    public function subscribe(callable $after)
    {
        $this->after($after);
    }
}

function watch($val = null) {
    return new Watcher($val);
}

$a = watch(42);
$a->subscribe(function($v){
    // Значение изменилось!
});
$a(23);