extending built in classes?

Hi - I am trying to extend a built in class, but cannot seem to get my subclass methods called. class movObj extends SWFMovie {

    function init() {
       echo("init");
    }

} $m = new movObj(); $m->init(); Gives::: Fatal error: Call to undefined method SWFMovie::init() ... This method call i would have hoped would pass up to the baseclass. But if I add a constructor to my own subclass, the init method can be called. BUT i can now not call any of the SWFMovie methods. Like, it is not really extending the built in class, or there is no hierarchy. I messed with various versiosn of trying to call the super myself: parent::method $this->parent->method() without much success. Is this a limitation of phps classes? We cannot subclass built in types (perhaps they were built/not compatible with php5 object model)? thanks!