Adding filtering to any class simply by having it extend the Filterable class.

filterable.php

<?php
// Builds off of Aspect-Oriented Design demo by Garrett Woodworth: 
// <http://phpadvent.org/2010/aspect-oriented-design-by-garrett-woodworth>

class Filters {
    protected static $_filters = array();
    
    /**
     * Register a new filter; note there could be an array index also provided
     * to indicate where in the list order that a callback filter should be applied
     */
    public static function add($member, $callback) {
        static::$_filters[$member][] = $callback;
    }
    
    /**
     * Run the method's return value by all of the filters in order of addition
     */
    public static function apply($value, $member, $object, $params){
        if(isset(self::$_filters[$member])){
            foreach(self::$_filters[$member] as $callback){
                $value = $callback($value, $member, $object, $params);
            }
        }
        return $value;
    }
    
    /**
     * Remove a filter
     */
    public static function remove($member, $callback){
        $i = array_search($callback, self::$_filters[$member]);
        if($i !== false){
            list($removed) = array_splice(self::$_filters, $i, 1);
            return $removed;
        }
        return null;
    }
}


/**
 * This is a wrapper class that gets inherited from to add filtering to any class method.
 * Function argument filtering could be added here as well, as well as instance
 * variable filtering.
 */
class Filterable {
    
    function __call($name, $args){
        $_name = '_'.$name;
        $return = call_user_func_array( array($this, $_name), $args );
        $key = get_class($this)."::$name";
        return Filters::apply($return, $key, $this, $args);
    }
    
}


/**
 * Sample class that is filterable. Note that all filterable methods merely need
 * to be prefixed by a single underscore, but then they are invoked without it.
 */
class HelloWorld extends Filterable {
    public function _greet($name){
        return "Hello, $name!\n";
    }
}


/**
 * Demo
 */

$hello_world = new HelloWorld();
print $hello_world->greet("John");
//> Hello, John!

Filters::add('HelloWorld::greet', function($value){
    return str_replace('Hello', 'Hola', $value);
});

print $hello_world->greet("John");
//> Hola, John!

Filters::add('HelloWorld::greet', function($value){
    return str_replace('John', 'Johnny', $value);
});

print $hello_world->greet("John");
//> Hola, Johnny!