How to implement the observer pattern in PHP

<?php
  //定义观察者调用接口
  class transfer{
    protected $_observers = array();
    //注册对象
    public function register($sub){
      $this->_observers[] = $sub;
    }
    //外部统一调用
    public function trigger(){
      if(!empty($this->_observers)){
        foreach($this->_observers as $observer){
          $observer->update();
        }
      }
    }
  }
  //观察者接口
  interface obserable{
    public function update();
  }
  //实现观察者
  class listen implements obserable{
    public function update(){
      echo &#39;now first time you need to do listen<br/>&#39;;
    }
  }
  class read implements obserable{
    public function update(){
      echo &#39;now first time you need to read<br/>&#39;;
    }
  }
  class speak implements obserable{
    public function update(){
      echo &#39;now first time you need to speak<br/>&#39;;
    }
  }
  class write implements obserable{
    public function update(){
      echo &#39;now first time you need to write<br/>&#39;;
    }
  }
  $transfer = new transfer();
  $transfer->register(new listen());
  $transfer->register(new read());
  $transfer->register(new speak());
  $transfer->register(new write());
  $transfer->trigger();