event.php

<?php

  class Event {
    private $observers = [];

    function __construct(string $event) {
      $this->observers[$event] = NULL;
    }

    function dispatch(string $event, ...$value) {
      foreach($this->observers[$event] as $fn) {
        if (is_callable($fn)) {
          call_user_func_array($fn, $value);
        }
      }
    }

    function attach(string $event, $callback) {
      $this->observers[$event][] = $callback;
    }

  }

  $event_push = new Event('push');

  $event_push->attach('push', function($value) {
    echo $value;
  });

  $event_push->attach('push', function($value) {
    echo 'second';
    echo $value;
  });

  $event_push->dispatch('push', 'hello!!');