observer.php

<?php

// observable user
class User {

    const EVENT_TYPE_AFTER_LOAD_FOR_UPDATE = 'afterLoadForUpdate';

    const EVENT_TYPE_AFTER_SAVE = 'afterSave';

    private $Foo;

    // list of observers
    private $observers = array();

    public function getFoo() {
        if (!$this->Foo) {
            $this->Foo = new Foo($this);

            // register new object as observer
            $this->observers[] = $Foo;
        }

        return $this->Foo;
    }

    public function loadForUpdate() {
        // do real load for update

        // fire event for each observer
        foreach ($this->observers as $Observer) {
            $Observer->handleUserEvent(self::EVENT_TYPE_AFTER_LOAD_FOR_UPDATE, $this);
        }
    }

    public function save() {
        // do real save

        // fire event for each observer
        foreach ($this->observers as $Observer) {
            $Observer->handleUserEvent(self::EVENT_TYPE_AFTER_SAVE, $this);
        }
    }

}

interface UserObserver {

    public function handleUserEvent($type, User $User, array $args = array());

}

class Foo implements UserObserver {

    private $User;

    private $queue = array();

    public function __construct(User $User) {
        $this->User = $User;
    }

    public function makeSomeoneHappy() {
        echo 'Screw you!'."\n";

        // queue some stuff for future
        $this->queue[] = 'We all gonna die!';
    }

    public function handleUserEvent($type, User $User, array $args = array()) {
        if ($type == self::EVENT_TYPE_AFTER_SAVE) {
            $this->doDeadlyStuff();
        }
    }

    protected function doDeadlyStuff() {
        // only say after real User::save
        while ($element = array_shift($this->queue)) {
            echo $element."\n";
        }
    }

}


$User = new User();
$Foo  = $User->getFoo();

$User->loadForUpdate();

$Foo->makeSomeoneHappy();
// will output: Screw you!
$Foo->makeSomeoneHappy();
// will output: Screw you!

$User->save();
// will output: We all gonna die!
// will output: We all gonna die!

// that's it!