Hi Julia,
and welcome to the boards.
If you want to load a movie, it should be a movie, and not a php script.
You can use any movie name you like, as long as the movie resides at the same domain.
You can even send this link to the movie dynamicly by the php script.
I suppose, that is what you wanted.
in this case, your IF-Structure could go into the php side.
<?php
....
if($userStatus==0){
$nextMovie="mainMovie.swf";
}else{
$nextMovie="bye.swf";
}
echo "user=".$nextMovie;
?>
at the flash side, it is only a single line

if(success) loadMovie(lvIn.user);