An Observer in a Tweet

Signal.php

<?php

/**
 * The Signal class is the smallest and fastest observer written in PHP 5.6 and
 * using some new features such as the variadic functions and the arguments
 * unpacking.
 *
 * ...and it fits in a tweet.
 *
 * @author Quentin Schuler aka Sukei <qschuler@neosyne.com>
 */
class Signal {
 private $c = [];
 function listen($c) { $this->c[] = $c; }
 function notify(...$a) { foreach ($this->c as $c) $c(...$a); }
}

<?php

class Follower
{
    public $name;
    public $onFollow;

    public function __construct($name)
    {
        $this->onFollow = new Signal();
        $this->name = $name;
    }

    public function follow(Follower $follower)
    {
        $follower->onFollow->notify($this);
    }
}

<?php

$neosyne = new Follower('neosyne');
$neosyne->onFollow->listen(function($follower) use ($neosyne) {
    echo sprintf('%s got a new follower: %s.', $neosyne->name, $follower->name);
});

$sukei = new Follower('sukei');
$sukei->follow($neosyne); // neosyne got a new follower: sukei.