Event-driven architecture and hooks in PHP

I am planning on working on a game that has a PHP back-end to communicate with the data repository. I was thinking about it and concluded that the best design paradigm to follow for our game would be event driven. I am looking to have an achievement system (similar to the badges system of this website) and basically I would like to be able to hook these "achievement checks" into a number of different events that occur in the game. ie:

When a user does action X hook Y is fired and all attached functions are called to check against an achievement requirement.

In structuring the architecture like this I will allow for new achievements to be added easily as all I will have to do is add the checking function to the proper hook and everything else will fall into place.

I'm not sure if this is a great explanation of what I intend to do, but in any case I am looking for the following:

1. Good reference material on how to code an event-driven application
2. Code snippet(s) showing how to put a "hook" in a function in PHP
3. Code snippet(s) showing how to attach a function to the "hook" mentioned in point 2

I have a few ideas as to how to accomplish 2) and 3) but I was hoping that somebody well-versed in the matter could shed some light on best practices.

Thank you in advance!

Good reference material on how to code an event-driven application

You can either do this with "dumb" callbacks (Demo):

class Hooks
{
    private $hooks;
    public function __construct()
    {
        $this->hooks = array();
    }
    public function add($name, $callback) {
        // callback parameters must be at least syntactically
        // correct when added.
        if (!is_callable($callback, true))
        {
            throw new InvalidArgumentException(sprintf('Invalid callback: %s.', print_r($callback, true)));
        }
        $this->hooks[$name][] = $callback;
    }
    public function getCallbacks($name)
    {
        return isset($this->hooks[$name]) ? $this->hooks[$name] : array();
    }
    public function fire($name)
    {
        foreach($this->getCallbacks($name) as $callback)
        {
            // prevent fatal errors, do your own warning or
            // exception here as you need it.
            if (!is_callable($callback))
                continue;

            call_user_func($callback);
        }
    }
}

$hooks = new Hooks;
$hooks->add('event', function() {echo 'morally disputed.';});
$hooks->add('event', function() {echo 'explicitly called.';});
$hooks->fire('event');

Or implementing a pattern often used in event-driven applications: Observer Pattern.

Code snippet(s) showing how to put a "hook" in a function in PHP

The manual link above (callbacks can be stored into a variable) and some PHP code examples for the Observer Pattern.