observer.php

<?php
    
  class Events {
      
    private static $listeners = array();

    public static function listen($event, $callback) {
      self::$listeners[$event][] = $callback;
    }

    public static function trigger($event, $params) {
      foreach (self::$listeners[$event] as $event => $callback) {
        call_user_func($callback, $params);
       }
    }
          
  }

  class ButtonElement {

    private $background;
    private $text;

    public function color($color) {
      $this->background = $color;
      Events::trigger('color', $color);
    }

    public function content($text) {
      $this->text = $text;
      Events::trigger('content', $text);
    }
  
  }

  class ButtonElementObserver {
    
    public function onColorChanged($color) {
      echo "Color changed: " . $color;
    }

    public function onContentChanged($text) {
      echo "Content changed: " . $text;
    }

    public function startObserver() {
      Events::listen('color', array($this, 'onColorChanged'));
      Events::listen('content', array($this, 'onContentChanged'));
    }

  }

  $observer = new ButtonElementObserver();
  $observer->startObserver();

  $button = new ButtonElement();
  $button->color('blue');
  $button->content('save');

?>