An SWFSprite is also known as a "movie clip", this allows one to create objects which are animated in their own timelines. Hence, the sprite has most of the same methods as the movie.
Note that when adding an sprite to a container (sprite or movie), only the object added to the sprite before the addin to the container will be displayed.
<?php
// In this case, myShape will be displayed...
$sp=new SWFSprite();
$container=new SWFSprite();
$sp->add($myShape);
$container->add($sp);
// but not in this case
$sp=new SWFSprite();
$container=new SWFSprite();
$container->add($sp);
$sp->add($myShape);
?>