cross scripting is when you load an swf as a child and the child swf can use parent swf functions during runtime and also the parent can call child swf functions on runtime, im going to leave this here because it's useful when making 3d games, when you have a 3d swf you might want to load it up in another swf container to have more perspective and control over the 3d window.

swf 1 name: swf.swf

PHP Code:
var my_var:String 'hello world';
var 
myBytes:ByteArray;
var 
new_player:Loader = new Loader();
var 
mp_package_ready:Boolean false;
function 
load_init_player(){
new_player.contentLoaderInfo.addEventListener(Event.COMPLETEinit_player_loaded);
new_player.load(new URLRequest("var_reader.swf"));
}
function 
init_player_loaded(e:Event):void{
var 
myBytes=e.target.bytes;
mp_package_ready=true;
loader.loadBytes(myBytesloaderInfoStuff);
new_player.contentLoaderInfo.removeEventListener(Event.COMPLETEinit_player_loaded);
}
load_init_player();

var 
loader:Loader = new Loader();
var 
loaderInfoStuff:LoaderContext = new LoaderContext(falseApplicationDomain.currentDomainnull);
loader.contentLoaderInfo.addEventListener(Event.COMPLETEcached_mp_loaded);
loaderInfoStuff.allowLoadBytesCodeExecution true
loaderInfoStuff.allowCodeImport true;

function 
cached_mp_loaded(e:Event):void{
var 
loaded_mc e.target.content as MovieClip
addChild(loaded_mc);
loaded_mc.callit();
loaded_mc.readParent();

swf 2 name must be: var_reader.swf
PHP Code:
trace('hey');
function 
callit(){
trace('hey from callit');
}
function 
readParent(){
trace(MovieClip(root.parent).my_var);