The observer pattern using PHP

The observer pattern gives you another way to avoid tight coupling between components. This pattern is simple: One object makes itself observable by adding a method that allows another object, the observer, to register itself. When the observable object changes, it sends a message to the registered observers. What those observers do with that information isn't relevant or important to the observable object. The result is a way for objects to talk with each other without necessarily understanding why.
A simple example is a list of users in a system. The code below shows a user list that sends out a message when users are added. This list is watched by a logging observer that puts out a message when a user is added.

Observer.php

<?php
interface IObserver {
function onChanged($sender, $args);
}

interface IObservable {
function addObserver($observer);
}

class UserList implements IObservable {
private $_observers = array();

public function addCustomer($name) {
foreach( $this->_observers as $obs )
$obs->onChanged( $this, $name );
}

public function addObserver( $observer ) {
$this->_observers []= $observer;
}
}

class UserListLogger implements IObserver {
public function onChanged( $sender, $args ) {
echo( "'$args' added to user list\n" );
}
}

$ul = new UserList();
$ul->addObserver( new UserListLogger() );
$ul->addCustomer( "Jack" );
?>

This code defines four elements: two interfaces and two classes. The IObservable interface defines an object that can be observed, and the UserList implements that interface to register itself as observable. The IObserver list defines what it takes to be an observer, and the UserListLogger implements that IObserver interface.