why is direct access to an array not possible using a magic __set() method?

Ok, so here's a snippet of my Properties base class, that is extended in most of the classes in my application:

class Properties {
    protected $properties_arr = array();

    /**
     * Magic function to be able to access the object's properties
     * @param string $property
     */
    public function __get( $property ) {
        if ( array_key_exists( $property, $this->properties_arr ) ) {
            return $this->properties_arr[ $property ];
        }

        return $this->getUndefinedProperty( $property );
    }

    /**
     * Magic function to be able to access the object's properties
     * @param string $property
     * @param mixed $value
     */
    public function __set( $property, $value ) {
        if ( property_exists( $this, $property ) ) {
            $this->setProtectedProperty( $property );
        }

        $this->properties_arr[ $property ] = $value;
    }

It's pretty basic, and there's nothing wrong with it, but I'm running into a problem that I've encountered a couple times before, and it's that you can't perform certain actions on an array property through the __get method.

Doing this, for instance:

$MyClass = new Properties();
$MyClass->test = array();
$MyClass->test['key'] = 'value';

you would expect the $MyClass->test array to contain one item, but it's still an empty array! Now I know I can work around it by just assigning an array with the items already in it, but I would just really like to know why this is (and if there's a better solution).

Thanks!